Per the protection FAQ, questions are automatically protected if:
- Three answers from three different low-rep users were deleted.
- Five or more answers from low-rep users were posted in the last 24 hours.
- Two answers were deleted by spam flagging.
The FAQ does mention that the first and third criteria above are implemented in a crude fashion, and links to another post that describes the true way they are implemented. However, that doesn't seem to describe what happened here.
The protection seems to have been caused by the deletion of that answer, so it can't be the second one. It was also voluntarily deleted by the author, so it can't be the third one either. Finally, the answer was posted by a high-rep user with 32.5k+ rep, so it's not posted by a "new user".
How did the question get protected after an answer posted by a high-rep user got deleted, and that too voluntarily?
I suspect that the "unique authors" check in the linked MSO post is not properly checking if the author is new, so this may be why the most recent contest's revision history has entire pages consisting of auto-protection/unprotection events.