Auto insurance at-fault decision reversed?

i live in CA; was in an accident with another motorist feb. 2010 and we both had the same insurance company (since the accident i am no longer a policy holder with them). the company determined that neither party was "at fault"--50/50 split down the middle--therefore insurance ended up paying half of my damages. i just got a letter today (over a year later) saying that they are reversing their decision and saying i was the at fault party and now they are trying to recover the damages for the other person. is this something they can do and how is this legal?