[BenjiWiebe]

There's wisps of atmosphere where they are. The drag causes them to slow down, which lowers their orbit. Then they hit thicker air and get more drag and go lower yet. And then they get so hot from all the friction the cook to bits.

Oversimplified.

[bryanlarsen]

Yup. There's not much atmosphere way up there, but if you hit it at 17,000 miles per hour even a little bit can be significant!