Looks to me like a clutch drop. But I'm no expert and don't know how you drive your car. Cryo-treated RA gears are nice though.

When you do a clutch drop (or a very very hard launch) on a WRX (non STi), the aluminum tranny case will flex a bit. This can cause the shafts to bend very slightly and gears to slightly move apart, and if the forces are strong enough, shear teeth. Something like that, anyways.

The pictures look like teeth just sheared right off. Look at the one gear with its missing teeth, and compare it to the smaller one that looks like they all came off. They may not mech together, but if so, it looks 1-to-1 on missing teeth.