I agree about the water being too hot can lead to exploding mice, but they can also explode if they have been defrosted and refrozen too many times. I'm not saying you defrosted and refroze them, but perhaps you got a bad batch that had been allowed to melt? When you defrost a mouse it should look pretty much like it just died, if the belly is distended or the skin there looks and feels thin then there is a good chance that they defrosted at some point.