Scott,
Thanks for the post. This is one that we used to get a lot of comments on but fewer and fewer these days.
The problem is most likely that the bolt was not tightened properly the last time the blades were installed or the spring disc washer needs to be replaced. If the bolt is not tight or if the washer is worn, weak or in need of replacement the bolt can self tighten. If any of these situations are present it may not take much of an impact to cause the blade to spin and self tighten. Even thick, wet, lush lawns can cause the blade bolt to self tighten if not installed properly.
If everything is working properly and you hit something the blade bolt may tighten some but not nearly as much as if the bolt was loose. A tight bolt clamps the metal components together and locks them before the weight of the blade can gain enough momentum to really tighten the bolt.
Since you've already broken a couple of sockets I would recommend taking the mower into a shop with a larger impact or getting out your hand held grinder and grinding off the head of the bolt. With the head removed the threaded section can often be removed with your fingers. I would also recommend replacing that bolt and the spring disc washers on each blade. This will help ensure that you don't have a repeat of the same situation.
Please let me know if I can be of any further assistance.
Thanks
Terry