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Looks to me like a loose primer pocket combined with a light load. Firing pin strikes the primer and pushes the whole cartridge forward in the chamber, pressure rises and unseats the primer first since it is loose. As pressure rises it seals the primer against the pocket. Pressure continues to build and sets the case back as the bullet begins to exit the cartridge and now since the anvil is free to float around in the extra space the primer ends up concaved since the firing pin is still pushing against the primer. Sometimes you will see this and the edges of the primer will be crushed also if the case sets back forcefully enough. If it was an average load it would have flattened the concave shape back out. A heavier load it would have just blown the primer out of the case. When you reprime the case note how much force it takes to put the new primer in. If there is little or no resistance you will probably want to trash the case.
Might be like everyone else has said too (snap capped). I can't see more than one pin strike in the photo though.
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