Stronghold jaws keep backing off

I have an interesting little stump that I cast in epoxy. I began shaving it down with my carbide cutter and all was going fine when all of a sudden a big grab and a chunk came off.

I switched to my bowl gouge and noticed it sounded odd. The chuck had lossened so I tightened it.

The grab and chunk coming off was due to the loose chuck.

Started again and a minute later it was loose again. Repeated over and over.

I've had this chuck for a long time and it has slacked off a few times, but this is another level.

When I tighten it I rotate to tighten from each hole and give it a pretty good turn.

I'm stumped at what is going on. I've turned larger and smaller pieces. I've turned pieces with epoxy and I've turned at different speeds. I have a good sized tenon. Any ideas what's going on? Do the chucks wear out?