I have noticed that my aim mode sometimes gets changed during combat without pressing ALT. I know I didn't hit it accidentally as I use an N52te game pad and don't have ALT mapped to any of the keys.
I finally was able to recreate the effect and found that some cover interactions change aim mode. I don't know if this is intended or is a bug. Going to guess the latter.

Clip of it happening:

https://youtu.be/cPm3q_vtpb4