what would cause a car to accelerate on its own?

Strange as it has a cable driven throtle body. Maybe the cables are used and stick sometimes, or maybe the idle air control valve on the body(used or the o-ring could be the problem). You could also have a losen clamp and have more air an necessary. Try spraying some spray near the throttle body and the hoses.

make sure the cruise control is off

Maybe a bunched up floor mat. In my estimation 99% of unintended acceleration is operator error.

I had the same issue, there was a recall on this vehicle for the accelerator pedal bracket causing the pedal to stick in hot weather. I used wd40 on mine and it helped. Probably would be better to replace the bracket. Operator error indeed.