On Thursday, December 19, 2019 at 9:48:33 AM UTC-5, Wade Garrett wrote:
Got an older model Genie screw drive garage door opener (IS, ISL, IC
series) that started reversing the lowering door when its bottom edge
reached about a foot from the ground yesterday. It happened when
activated by either the remote transmitter or the wall switch

I know that if the object-sensor safety electric eye is obstructed, the
door reverses immediately after it starts down-- but I can't figure out
what would cause it to reverse at the one-foot level.

The opener has worked fine for years. The stop-activation levers for
raising and lowering haven't moved and are in the correct place, I
haven't touched the closing force or other adjustment screws on the
motor head, and the tracks and rollers appear OK too.

Ideas?

Have you looked for a leaf or similar hanging from the bottom of the door?