cryopurge

I am running a server for my friends, a vanilla survival with no plugins, and i am having issues. Every so often, other people get kicked, or made to leave, without me doing anything. This has never happened to me. I am not using any moderator plugins. They are able to join again straight away, but this is still very frustrating. My friend says that he gets kicked out to the minehut hub. Is there any way you could help?