It could be the ring gap.
When the rings are in the cylinder, they form an almost complete circle. There is a small gap keeping them from being a full circle. As the engine comes up to running temp, the rings expand from the heat. As they expand, that gap closes. If it closes all the way, and the ends of the rings butt up against each other, there is no room left for expansion.
The rings become forced against the cylinder wall, and cause the piston to stop.
On high performane engines this can cause catastrophic engine failure, because the piston is moving too fast to stop, and the top off the piston will get ripped off.
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