I had a valve evulsed from a 451 tube this summer. The first time I can remember it happening. Mine was an old tube. Possibly not serviced since buying the bike a couple of years ago.

I might have been able to glue it back together in an emergency, but managed to put a 700c tube in the tire.

In my case, the tube was jammed into the tire/rim quite hard. Otherwise, I don't remember anything out of the ordinary.

Is it possible that your rims were drilled Schrader, and you're using Presta valves? Other risk factors?