Is that the only crack? The reason I ask is that it appears to be at the rear bridge. This part of the action isn't subjected to nearly as much stress as the front bridge because the front of the action (where the barrel screws in) is where the bolt lugs engage locking lugs in the action itself.
I'd say that the action was improperly tempered, left too hard, and subjected to stress of some sort, but not necessarily an over-pressure round.