Either at a Hydra party or post-CW where Bucky is living in Stark tower, but someone has it out for the Winter Soldier due to a past incident. And when they get their hands on the control codes, they use them to make Bucky hurt himself... badly. Ideally looking for pain/physical hurt, but not sexual.

+Steve finds out about all this and angsts and/or feels the rage
++Bucky can't remember what the incident was
+++Bucky almost dies