(Reuters) — Josef Ackermann resigned because he saw the suicide of Zurich Insurance Group Ltd.'s finance chief curbing his ability to drive through change to revive profits, a source close to the former chairman said Sunday.

Top managers at the Swiss insurer, now battling to reassure investors, told Reuters they were puzzled by Mr. Ackermann's abrupt departure after less than 18 months in the job and by his explanation that Pierre Wauthier's family blamed him for driving the Zurich CFO to kill himself — an allegation Ackermann denied.

Some speculated on other possible motives for Thursday's resignation, which surprised many who have followed the dynamic career of the 65-year-old Mr. Ackermann since he rose to prominence and power in turning around Germany's Deutsche Bank A.G.

But the person familiar with Mr. Ackermann's reasons dismissed talk of him using Mr. Wauthier's death a week ago as an excuse for a move motivated by other reasons — such as personal legal headaches or an existing disillusion with a company in which some say the banker was lost in the alien culture of insurance.

"He was not satisfied with Zurich's performance, which is why he campaigned to change this," the source said.

"When the CFO took his life, it became clear to him that he could not keep up this pressure. Because of this, the chances of succeeding were called into question. That's why he resigned."

Mr. Wauthier, 53, explicitly blamed Mr. Ackermann for putting him under pressure in a suicide note, his family told the company — an allegation Mr. Ackermann called "unfounded" as he announced he was leaving to spare the reputation of Europe's No.3 insurer.

The remarks on Sunday point to more fundamental concerns about Zurich's business strategy, with Mr. Ackermann apparently unhappy about management performance but realizing that being seen to pressure executives for improvement was unlikely to be an option in the wake of Mr. Wauthier's death.

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The initial dual messages from Mr. Ackermann — that he was stepping down over the family's allegations about his role in the suicide but also rejecting the accusations — had prompted head-scratching among senior managers at Zurich Insurance.

After a decade running Deutsche, why did he quit after becoming Zurich chairman only in March 2012, they wondered. Some added a note of disapproval of what it meant for the business.

"When the company is in crisis you need stability, not a chairman who takes off," said one person who has spoken to high-ranking managers.

"People are wondering if the Wauthier suicide is enough of a reason to resign, and people are wondering what other explanations there could be."

Chief Executive Martin Senn insisted to Swiss weekly paper NZZ am Sonntag at the weekend that there had been no evidence of tensions between Messrs. Ackermann and Wauthier.

But people in Mr. Ackermann's entourage vigorously reject the suggestion that he may have been motivated to step aside by factors other than the chief financial officer's death.

If concerns about what it would mean for future performance were the main explanation, however, it was no secret Mr. Ackermann had been struggling already to get his way at the company.

Even before Mr. Wauthier died, there were signs that Mr. Ackermann, who rose from the hard-charging world of investment banking to become one of Europe's leading economic power brokers, was struggling to change the cautious, consensual culture at Zurich.

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Shaking up that approach was one of the main reasons he was made chairman last year. But delivering change was not easy, and Mr. Ackermann encountered resistance from the start.

There was grumbling, insiders say, with Mr. Ackermann's style. He retained the services of his own spokesman from his Deutsche days and insisted on being given an office with a lake view — something his predecessor Manfred Gentz did not have.

At events like the World Economic Forum in Davos, Switzerland, Mr. Ackermann sometimes appeared to upstage the Zurich CEO, Mr. Senn.

Some within the company saw him as a fish out of water — a brash banker in the sedate world of insurance.

"Ackermann's mission when he came was to shake up Zurich, to infuse a more dynamic mentality into it," another source close to the departed chairman said. "Yes, insurance isn't banking, but there was still more 'oomph' to be wrung out."

Mr. Wauthier's suicide may have convinced Mr. Ackermann that this sort of change was impossible.

Another hypothesis, put forward by a senior executive at the insurer, was that Mr. Ackermann may have been worried about other scandals from his time at Deutsche Bank catching up with him.

Mr. Ackermann's resignation came shortly after German media reports that Munich prosecutors had summoned him and several other former Deutsche executives for questioning in a decade-long dispute with the heirs of deceased media magnate Leo Kirch.

Prosecutors want to examine claims, according to these reports, that former management board members at Deutsche misled the court during an earlier trial. Munich prosecutors and Deutsche Bank have declined to confirm the summons.

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People close to Mr. Ackermann have said the long-running Kirch affair has absolutely nothing to do with his resignation.

Meanwhile, the Swiss insurer is trying to return to business as usual. But it faces major challenges that go beyond the recent upheaval in its executive suite.

Two weeks ago, Zurich warned investors it would be hard pressed to meet certain targets after high natural disaster payouts led to a 27% drop in second-quarter net profit.

The firm is struggling to meet cost-cutting goals at its U.S. unit Farmers, and like other insurers is struggling to generate strong returns on investments.

Its shares have held up well since news of the suicide emerged.

The stock has slipped just 1.36%, broadly in line with the European Insurers index.

Still, over the past six months the company's shares have fallen 10% compared with a near 9% increase in the European sector.

And Mr. Senn told the newspaper that reaching the company's three-year profit goals would be difficult: "The death of Pierre Wauthier and the resignation of Joe Ackermann have tarnished the very good reputation of Zurich, without question," he said.

"The resignation of Ackermann was a personal decision. We have to accept that. I'm now working so we can blow away this loss of reputation, these clouds that have gathered over the company."