Unless there is a glitch on discord or on your client there is really only 3 possible explanations.
1. You have been kicked from the server
You can get kicked without even getting notified in dms. The server can just disappear from your server list and you have to join back in. I would recommend asking the moderators why you were kicked and try to solve it with them.
2. Your account has been breached
Someone could of logged into your account and left a couple servers. If this is the case, its probably a sibling or a friend trying to be funny.
3. A bot is joining/leaving servers for you
If you ever logged into a website with discord there is a permission that lets bots join (not sure about leave) servers for you. In order to double check if this is the case go to settings>authorizedapps and then check what permissions bots have and remove any if necessary.