Cam seal popped out... What could cause this?

You just answered the question in the last sentence! When the timing belt was replaced the seals were replaced also, and the technician did not seat/install the seal properly and the internal crankcase pressure caused the seal that was not installed all the way was the weak link and it popped out (have seen this numerous times) ADDED: It is normal practice to replace the cam and crank seals when the belt is done but, If you are sure that is the case Then the first thing to check is for restricted PCV flow or a clogged PCV valve!