Ha!

I know on "The Garden's Spire" mission if you kill the end boss quickly enough the dropships for his reinforcements get stuck like this, so I'm wondering if this happens when you kill the Fallen Captain right away (I forget his name, but it's a major)? TBH, I don't know if he's in this area from the beginning or if he comes in on a later ship. I don't usually venture toward his area until the ships stop coming.

- No text -

...is the immediate "Guardian down!"

I know that's not it, but I like to think some part of the ship got knocked off and slammed him into a fine paste.