Cheap pans. Thin-walled that doesn't distribute heat evenly. So hot spots burn the pasta to the walls. Secondly, not stirring at all. Boiling will do some circ

Cheap pans. Thin-walled that doesn't distribute heat evenly. So hot spots burn the pasta to the walls. Secondly, not stirring at all. Boiling will do some circulation, but you have to keep some amount of stirring to prevent a small sticking turn into a burned to the pan problem.