There is no crawling in this game, so I expect that you have the AricsExpansion mod installed. Is it the itch.io version 1.8d? Do you have any other mods installed besides that and the Bugfix mod?
There does not appear to be any significance in them wearing the Pet Suit. If they are wearing handcuffs or have been assigned the Pet Behavior rule in the Customization tab, then they would be forced to crawl. Otherwise low energy or an increase in breast size can force a slave to crawl; the latter can be fixed by increasing strength or endurance.
Crawling slaves cannot be assigned any occupation outside of the mansion.