Auto Insurance Claims: California non-contact collision, rush hour traffic, hov lane

Question
The state is California, I was cruising down the freeway in the HOV lane in rush hour traffic.  A motorcycle was traveling on the line in between the hov lane and the first non hov lane.  I crossed over into the first non hov lane to the right and the motorcyclist went down on his bike.  My car didn't hit the motorcyclist. The police came and I wasn't issued any summons or anything like that.  I am not sure what was included in the police report.  This happened in August 2011.  I recently (02/12) received a letter from his insurance company stating I am responsible for reimbursing them at 80% of what they provided to the motorcyclist. Am I responsible for reimbursing his insurance company and what actions should I take in regards to notifying my insurance company?  I had coverage on the rental car but since there wasn't any contact, their insurance company said I shouldn't worry about filing a claim.  Any information you can provide is greatly appreciated.

Answer
You should immediately open a claim with your insurance company and the rental agency.  Fax the letter to both.  Depending on the law in CA, one of them will be primary and the other excess for liability coverage.  Both will investigate and determine if they agree that you were 80% liable.  The insurers will then try to work this out.  It is possible the case will go to arbitration or court, but you should be defended by either your insurer, the rental company self insurer, or both.  Cooperate fully as they investigate.  Explain the late notice by stating you did not think you were liable as there was no contact.