It's a surprise to see Bellatrix cower, but this isn't truly Bellatrix, it's her Trix and even though they haven't known each properly for long, rage controls her when she sees the deatheater vulnerable, it hurts her to see her like that and Terra finds out Hermione's statement from before was correct, Bellatrix would be the one to drag her along with her off the edge, although it's only partly correct now because they're different. She's now Terra, not Hermione and Bellatrix is now Trix, because anything, but those nicknames feel so wrong to her.