Cylinders that seize around the exhaust port of the cylinder usually come uner the headind of a lubrication break down ( loss of an oil film between the moving piston and the cylinder bore ) once the oil film is lost, the piston heats up very quickly and expands in the bore, the alluminium is then scraped off the front of the piston, this will gum the piston ring in, the hot gasess from the combustion process then blows past the piston, so the machine has basically seized.
Reasons for a lubrication seizure are.
Insufficent two stroke oil in the fuel.
Poor quaility two stroke oil in the fuel.
Incorrect grade of oil in the fuel.
Lack of maintenance.
Internal bearing break down within the machine.
Over heating due to dirt blocking starter cover air vents, and choked around cylinder finns.
Over heating due to poor chain maintenace ( blunt chain )
Incorrect carb settings ( running too lean )
Blockages in the fuel system causeing over reving.
Working practice ( over loading the saw ).
Air leaks within the machine causeing lean mixture and over reving.
Stale fuel, water in the fuel, dirty fuel.
Hope this answers your question, as you can see there are many reasons for a machine to seize.