If a player or mob stands inside the amor stand the armor stand will get "knocked back" when you hit the player/mob (sweep attack) .

https://youtu.be/AYMpSQw5yP0

Why is this a problem?
This is especially a problem on modded servers that uses land protection plugins like world guard or grief prevention since this bug prevents them from protecting armor stands, players can move these around and even out of the protected area.