Their not light infantry; they are clearly classified as line infantry...Ive seen Elite Guard units do this as well for some reason and I don't know why they are doing it.

How would I be able to remove and delete this ability for this unit? Where would I go within the mod files to actually delete the ability? I just want them to skirmish normally during a battle.

What their doing is fire by rank or advanced fire by rank in terms of technique in the above picture; have you ever seen this been done within vanilla?