This is happening to almost every server. It's mass leveling. If you are making a group post and getting 10 people and they are joining and leveling and constantly causing stops the server will reach it's max and shut down. It basically enters a dormant state until you force stop and restart it. This is up to a 30 minute process.
I don't believe this is correct. I have 2 people playing on my server, and this lock-up and restart happens well before we can get any leveling in. ...so not sure why you would think this is the issue. My server is pw protected, so no-one is joining randomly either.
My server is a public server I usually get 10 or so people joining at once and leveling when I make a group post about it. Then the server just derops -_____-