I'm not exactly sure WHAT caused this to happen. I was in a fight where I was Goku against Nappa and Vegeta. When Vegeta and Nappa swapped, Nappa only had a tiny bit of HP left. As he was in the middle of swapping in, I launched an energy bullet at him, and when it hit him, he transformed into this glitched character.

My best guess is that the transformation was triggered because he lost all of his HP as he was swapping in? After all, Nappa has one of those "transforms when at low HP" forms, so it might have somehow triggered a second time.