A lot of people are having this issue, the reason is simple, as Amnotanormalkid said, the Steam Sales made a lot of people buying the game and joining the servers.
If you are disconnected, just stop your truck somewhere safe and wait for the game to reconnect you in your current server. Sometimes like me at this moment, you'll have to restart the game and choose another server.
Just be patient, it's a mod in Alpha stage made by fans, I'm pretty sure that they're already working on improving server stability/capacity!
Happy trucking and of course, happy new year! ;-D