i'm not a guru ... but i think i know of some of the ways an engine can seize. basically, an engine is seized if it's locked in some way and can't move. usually, i think, it's because the piston is stuck in the cylinder.
for example, i have a garelli engine that is seized in a horrible way. at least one of the rings broke, and is jammed between the piston and the cylinder. that's more than seized, that's probably ruined.
but i think going far too fast (or not using 2-stroke oil in your fuel) can lead to engine seizers if the piston gets hot enough and not lubricated enough that it more or less "welds" itself onto the cylinder.
depending on how bad the seizure is, you can fix it. so don't loose hope if your engine is seized.
also, i suppose an engine could seize internally if the piston arm's main bearings were horribly damanaged in some way.