It's not hard to pop those out when replacing a ball joint or even strut. So if anything like that was done recently, that's likely the answer. I don't think I've seen one pop out on their own.
I would recommend replacing both the front differential gear oil (as it likely has some metal shavings from the slipping splines, and the spider gears likely got hot from spinning with only one axle connected), and ATF (as the transfer clutches got abused) after driving it like that.