That looks exactly like the "riveting" (or whatever you want to call it) that happens when a primer backs out of the case a little, due to a slight amount of what might be called "extra" headspace--though it really isn't, at least according to SAAMI.
As mentioned in the article, SAAMI headspace specs usually allow for about .007" of variation in chamber/ammo dimensions. At typical modern bolt-action pressures, this often results in "flattening" of primers, due to exactly what you posted: The primer backs out a little before max pressure is reached, with the rear of primer expanding due to not being supported by the primer pocket.
However, the case head is not necessarily "harder" brass than the primer cup, just thicker.
