Chris Forsberg of ESPN Boston and Doc Rivers have the answer:
Boston led by 11 points with 9:17 remaining, and while its offense certainly downshifted, its typically unwavering defense did too. Maybe even more so.
Of the 24 points the Lakers scored in the fourth quarter, a staggering 18 came in the paint. (How many points did Boston score in the paint in the fourth quarter? Zero.)
Considering the Lakers added four fourth-quarter free throws, only one basket came outside of the paint -- Bryant's winner. What's more, Los Angeles generated only two fast-break points in the period, meaning the Lakers simply brought the ball down the court, got it inside the paint, and scored.
"I don't know if concern is the word, but we haven't been able to get timely stops to end the game," said Rivers. "If anything bothers me, over the turnovers and the offensive [struggles], it's that we've been able in the past to lean on our defense when we go cold offensively.
"The Atlanta game, down the stretch, they scored every time. The Orlando game, down the stretch, they scored every time. And again tonight. The biggest quarters over the last three games, offensively, have been the fourth quarters by the other teams. And we have to reverse that."