It's mostly the friction: The spacecraft is going VERY fast (appr 30k km/h) when reentering the atmosphere, so that is the relative speed with which air molecules are hitting the surface. I won't go into equations on how much heat energy each collision produces, but as the craft falls deeper into the atmosphere it becomes denser, so there are more and more collisions.
Friction is the main way for the multi-ton spacecraft to slow down (from 30k to ~200 km/h), and that braking energy is focused on a relatively small surface area. A parachute is used only towards the end of the fall.
I found this on the temperatures that spaceships experience during reentry:
What are the top temperatures occurring during reentry?
__________________
e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread