As Imam18 said, it's still in alpha testing so will have bugs such as this. If it's stuck in the "game finishing" mode and won't let you join for around 5-10 minutes feel free to ask around in the shoutbox (the chat here on the website) to see if a staff member such as myself can come reboot it for you
I believe this is actually caused by a bug. If you want to avoid it from happening, do not leave the assault servers when a team is ~5 kills away from winning. What happens is at ~55 kills, the server is locked, so that way new players can't just join for the last few kills on a winning team to get a quick 300 xp. However, if everyone leaves, no one can complete the game, since no one can rejoin that close to the game finishing. And since the game will never finish, if no two players on opposing teams are there to get kills, the only way to restart a game is via reboot. TL;DR, don't leave assault games when a team is close to winning.
