It doesn't even have to be a totally "random" (unrelated) motorcycle accident.

If he panicked after the Stuxnet attack, as his family is reported to have said, then it's likely he has behaving erratically and was fearful for his life.

That could easily translate to circumstances where he rides a motorcycle in a particularly dangerous manner - e.g. fleeing from someone he thought was Iranian/Dutch/US/Israeli intelligence (even if they weren't).