It is most likely the attorney who issued the subpoena wants to know what you saw and heard at the time of the accident. Witnesses are often asked to testify if there are two different versions of what happened, and a neutral person is needed to describe what happened. You could probably give the attorney a call and discuss what you remember from the accident.
I agree. If a lawsuit is filed against you, it is in your best interest to contact a lawyer immediately. Even before a lawsuit is filed, you should avoid speaking to the adverse party - whether that is the driver of the other vehicle or his/her insurance company. Anything you say will likely be used against you. I practice in Washington state, but it's my experience that lawsuits are typically filed in the state where the accident occurred, so I imagine you will need to find an attorney in PA.