Rings can loose their tension and allow
oil or compression to pass by them, overheating is the most common reason. Many racing rings are designed to have a lighter tension and use gas porting to help expand the top rings. Aluminum engines have steel sleeves because the aluminum is just to soft. Sometimes carbon make rings stick in the ring-lands of the piston. The groove accepting the ring on the piston (ring-land) becomes deformed from pinging, heavy detonation or even breaks the rings so they fail. Some rings just never seat or seal against the cylinder walls.
When you think about the speed a piston must travel it is a little amazing. A piston in a 4.0 inch bore at 6000 RPM travels 66.6 feet every second but only gets 4 inches before it has to change direction.