And on a 12" sander, the center of the disk is spinning significantly
slower than the edge -- so don't hold the piece so close to the edge
of the disk.

dadiOH wrote:

Yes. The wood is getting hot. It is getting hot because of friction.
The friction is caused by you pushing too hard against a disc that is
(probably) spinning too fast. (I say "probably" because many disc
sanders spin at around 3500 rpm; ones that spin at half that do better.)

The cure is to use a much lighter touch.