And to add to TJ's...

Use freaking loctite on important bolts. The bolts may not have stripped out; they could have just simply loosened up over time. There shouldn't ever be any force in the sideways direction; at least not nearly enough to strip out the bolts, considering they're perpendicular to the bumper... My guess is they loosened up a bit, 'cause there wasn't a lock washer and/or loctite involved.