The only reason I can see to involve myself in that type of situation, is if someone were hurt. I would hope then to be able to offer some sort of assistance with my background as a firefighter/EMT. If there are no injuries, and you have no information to offer authorities as to what caused the accident, then you will only be in the way by sticking around.
However, if you do see something relevant, you should report it to the police. Here's an example, which by the way is true.
I was driving down a busy 4 lane road, and saw an accident occur. It didn't appear that anyone was injured, so I continued driving. Out of pure reflex, I looked in the rear view mirror as I passed the two cars, and saw that in the second car (which had rear ended the first) the driver and passenger were switching seats! The man driving got into the passenger seat, and the female passenger got behind the wheel. I also have a background in law enforcement, so to me, the only possible reason for this was for him to avoid a charge. Either he had no license, or warrants on his license. Either way, a big legal no-no. So I phone the local authorities and reported what I saw. If you feel uncomfortable giving your personal information, then ask to remain anonymous. They will investigate all reports regarding the incident, and your information may make the difference in their findings.