Several things can cause engine to seize up. Overheating condition could cause a piston to seize in cylinder, lack of good oil flow could allow a bearing to seize. (have seen oil pickup screens stop up with sludge serveral times on this engine)and possibly a sprocket could fail allowing timing chain to jam up and seize engine. I am curious of the mileage on this engine and what the conditions were when it stopped. Were you driving at highway speed? Had there been any noise prior to it stopping? How often did the oil and filter get changed?