Moderators change titles when they break the site guidelines. The relevant guideline is "Please use the original title, unless it is misleading or linkbait; don't editorialize."

https://news.ycombinator.com/newsguidelines.html

The submitted title was "Wall-climber robot spraypaints big murals, any idea how its hardware works?", which is actually more baity than the original (wrong direction!). It's also the sort of thing that will strike HN readers as transparently promotional, prompting them to flag a submission.