This is commonly called a case head separation. Most likely cause is an area of the cartridge case that was weakened for some reason (being .22 rimfire I would assume a manufacturing fault of some kind), but it could also indicate a chamber that does not fully support the cartridge case when fired.

Not a common occurrence but not highly unusual either. I would be looking carefully at fired cases to see if there are signs of unsupported areas of the chamber. If not, I would take this incident as a case failure with that particular lot of ammo in that particular rifle.