I’m not sure if this can happen on any map but I was playing Citadel and it was the third part of the map where the slaves need to be freed. I ran out of spawn and I “fell”. I think I tried to stab an enemy knight as I went off the ramp and my character’s arms started flaining. This lasted maybe 3 seconds until the knight killed me, I could not move or swing my weapon.
Looks like you entered ragdoll mode. It is an extremely rare bug not specific to Citadel… it usually happens when someone lands on you or mauls the crap out of you :)