In a letter [2 pp. pdf] posted on the diocese's Web site, Bishop Robert H. Brom said the diocese made its decision because any damage awards in the earlier trials could deplete "diocesan and insurance resources" and leave nothing for other victims.
That does not actually explain the timing. The diocese could go through trial and, if the trial resulted in a judgment against it, file for bankruptcy then. Hence, others offer alternative explanations.
"For three years they've told people they want to settle, they want to be transparent," said John Manly, a lawyer for a plaintiff whose lawsuit is scheduled to go to trial in April, "but the moment it became clear the truth will come out through a jury trial, they sought to shut down victims' ability to get compensated and get out the truth."
San Diego is the fifth U.S. diocese to file for bankruptcy.
Update: Commentary at Shark and Shepherd.