Why do tornadoes form when continental and a Polar form?

The terminology here is a bit confused. In weather there are 4 basic types of large-scale air mass: Continental polar air masses come off the land in cold regions and are cool and dry. Maritime polar masses come off the ocean and are cool and somewhat moist. Continental tropical air masses come of the land from warm regions and are warm and dry. Finally maritime tropical masses come off of warm oceans and are warm and moist.

When a continental polar air mass pushes into a maritime tropical air mass it forms a cold front. Since cold air is denser than warm air, the tropical air mass is forced upwards. This can trigger thunderstorms. Under the right conditions these storms can produce tornadoes.