What burned my cookies?

I made sugar cookies yesterday and the bottoms burned before the tops even got any color, and the sheet was on the center rack.
The only variation that I did was where the recipe said to put the balls of dough on an ungreased cookie sheet, I put a piece of non-stick foil on the sheet just to be sure they wouldn’t stick. Could this one piece of foil have caused the bottoms to burn?

Aside from that, they were soft and delicious. Just the stupid bottoms!