High temperatures due to insufficient lubrication, an extremely lean mixture, contaminated intake air, abusive riding techniques, riding downhill with the throttle closed all contribute to a seizure in the end.
The piston actually starts to melt and the aluminum attaches itself to the cylinder causing the piston to stop. In a serious case the piston welds itself to the cylinder and you must replace both. In most cases you can reuse the cylinder after removing the embedded aluminum with muriatic acid and honing the bore. If you find scratches in the bore, then it's rebore and new oversize piston time.