One of two things caused that to occur. One is the bolts are too long and do bottom out. But you said that wasn't the case. The other is if the insert is mounted slightly below the surface of the riser/finish. Then, even with a washer, the bolt will pull the insert to the washer, which is flush with the surface, usually bringing some wood along with it. I suppose there is a third possibility, and that would be that the insert wasn't properly epoxied in place to begin with and just worked its way loose through shooting.
Judging from the bad fixes I've seen on a lot of used bows, it occurs on a lot of other brands as well. Mostly, it's folks using bolts that are too long. That will pull out the inserts regardless of how well they were put in.
I just fixed one on my brother's Bear T/D riser. Not an easy fix. Given the construction of the insert, about impossible to get the old one out without lifting some of the surrounding wood. Had to do that, work some epoxy around the perimeter of the hole, re-drill it and re-epoxy a new insert in place.