I also use TrackIR, head movement input, however nothing within TrackIR seems to be creating any spurious input.
This bug seemingly occurs after more than 15-30 minutes of game play, likely indicating other users are joining with modifications, causing server anomalies. This is very typically/commonly known bug with ARMA 3.
If I were paranoid, I'd almost say somebody is watching through my player's eyes (another common/old/fixed ARMA 3 bug), and possibly inadvertently activating their scroll wheel mouse down input action. Usually though, if this were the case, other keyboard input would also be occurring.