It doesn't just happen at the top of a loop. It can easily happen at any point in a loop or steeply banked turn if you have enough elevator travel and the balance point is such that there's enough control to force the wing to the stall point. It can happen at slow or high speeds depending on how the model reacts.

I had a model that would do the first loop fine with full elevator and after that it slowed down but the loop tightened up. At tlat point the model was "hiccuping" around the loops unless I opened it up by easing off the elevator.

Racing models sometimes need to be tuned for elevator throw to avoid pulling too much in the near vertical banked turn and having the wing stall and snap the model out of the turn.