In my experience with this type of betting it seems to be done by weaker post flop players. I think the basic theory is that if they bomb the pot on the flop they can fold out non-top pair 1 pair hands and all draws (except nutted combo draws) regardless of whether they actually have it or not themselves, they are basically trying to get you to fold 100% of the time on the flop. Sometimes you just have to reach down cop a squeeze of your balls to see how big they are today and go with your gut feel. The big bet on the flop is often a bluff or semi bluff. 

However, on the turn or rvr I would be a lot more wary of calling off light as these are much more likely to be over bets for value and much less likely to be bluffs.

Its a frustrating play to face on a frequent basis to be sure, but its No Limit Holdem and thats what makes it exciting.

You can use this play to your advantage as well, if it is a regular play on the site you play on it means other players are adjusting to it. You will probably get paid off much wider on the flop when you are nutted. What I am saying is to do the opposite to what most players are doing i.e. over pot bet for value on the flop (unlike villein who is using it as a bluff technique). 

Good luck