That’s a private server thing, the original way it works is that when you are sitting you will receive double damage, the combat log shows a crit.
Only if timed correctly on classic can you stand as you get hit and receive this fake crit, that will proc crit abilities, and is in fact a bug, some streamers got it to sort of work so it needs to be looked at. Spell batching probably?
“Being critically struck while using /sit to sit does not cause abilities like Enrage, Blood Craze, and Reckoning to activate.”