Last fall, senior partners at Kaplan Hecker & Fink, a New York law firm known for championing liberal causes, made a fateful decision: They were going to sideline their hard-charging and crusading founder, Roberta A. Kaplan.

The reign of one of the country’s most prominent lawyers was coming to an end.

Ms. Kaplan was already famous when she founded her law firm in 2017, having won a landmark Supreme Court case that paved the way for marriage equality for gay Americans. The firm soon gained national prominence because of her leadership in the #MeToo movement, and more recently for high-profile victories against white supremacists and former President Donald J. Trump.

But those triumphs couldn’t overcome an uncomfortable reality, according to people familiar with the law firm’s internal dynamics.

In the eyes of many of her colleagues, including the firm’s two other named partners, Ms. Kaplan’s poor treatment of other lawyers — ranging from micromanagement to vulgar insults and humiliating personal attacks — was impairing the boutique firm she had built, the people said. For one thing, they said, she was jeopardizing its ability to recruit and retain valuable employees.

Ms. Kaplan and other partners had also clashed over issues of management and strategy, and some of her colleagues were frustrated by the difficulties of achieving consensus with her, several people said.

Ms. Kaplan was told last fall that it had become untenable for her to remain on the firm’s management committee — a sharp rebuke for a founding partner. She agreed to step down from the committee. The decision began a monthslong chain of events that culminated this week with Ms. Kaplan’s announcement that she was leaving Kaplan Hecker to start a new firm.