I assume you mean pump action and not break action!
Anyway, it's pretty common for some pump shotties to kick partially open after a shot, especially with heavy loads. Especially if you are holding back on the stock, which is of course natural and proper way to do it to get it firmly against shoulder.
As long as you can't open breech while pulling back before you pull the trigger, it's safe enough AFAIK. Don't know the cure, unless it would be replacing worn slide bars (or whatever you call them) on the forestock or something, though.
But some folks report their brand new Mossberg pumps doing this, so it may not be worn parts at all, but just inherent nature for that gun.
Better than some of the current runs of 870's, where you can't hardly get the damn hull out at all after firing some rounds.
- OS