Ive seen this on a front axle....
the stub side of the CV let go (which in and of itself is unusual its normally the axle from the diff to the CV that goes.) and flared out the axle inside the stub, the metal from the hub side embedded itself inside the stub and locked up the drive flange. Fortunately we were able to pull the hub off the bearing because there was no way the inner bearing was coming out over the bulge in the hub.

