Someone always loses an arm in the Phase 2 movies

This is a deliberate homage by Kevin Feige to Star Wars. In that saga, Anakin, Luke, Vader, C-3PO, Mace Windu, Count Dooku and many more lose a limb.

In Phase 2, the same thing happens to someone in every single movie. Thor loses his arm in Thor 2, Aldrich Killian in Iron Man 3, Bucky in The Winter Soldier, Groot in Guardians of the Galaxy, Iron Man in Age of Ultron and Yellowjackets in Ant-man.