The code for the hot post selection can be found in the app/controllers/application_controller.rb file (search for hot_questions).
Eligible for Hot Posts are all Questions or Articles that
- have any activity in the last seven days,
- are in an eligible category (can be changed per-category) and
- have a score over a per-community score threshold
Sorted by descending score. The maximal number of top posts (number configurable per-community) are chosen for the board.
And about the specific post you mentioned: There was recent activity, namely the deletion of one answer by its author, which is apparently enough to trigger "activity in the last seven days".