Ever since JPMorgan Chase disclosed a multibillion-dollar trading loss this month, the central mystery has been how a bank known for its skill at risk management could err so badly.

As early as 2010, the senior banker who has been blamed for the debacle, Ina Drew, began to lose her grip on the bank’s chief investment office, according to current and former traders. She had guided the bank through some of the most rugged moments of the 2008 financial crisis, earning the trust of Jamie Dimon, JPMorgan’s chief executive, in the process.

But after contracting Lyme disease in 2010, she was frequently out of the office for a critical period, when her unit was making riskier bets, and her absences allowed long-simmering internal divisions and clashing egos to come to the fore, the traders said.

The morning conference calls Ms. Drew had presided over devolved into shouting matches between her deputies in New York and London, the traders said. That discord in 2010 and 2011 contributed to the chief investment office’s losing trades in 2012, the current and former bankers said.