The reason you were kick is because someone who was a donor (or higher, i.e. moderator/admin) joined the game as it was full. This system works by kicking the last person to joined the server.
The reason it is like this is because sometimes staff members NEED to join a game to watch for hackers etc. The reason donors automatically kick the last person who joined the server (full) is because they're the reason we have servers, so we decided to let them join whenever they want. This will never change and the only way you can prevent this is to join servers quicker, donate or become a staff member.
If you wish to learn more about donating/donors, or would like to yourself, please click
here.