Site turned to maintenance mode without changing any settings I have a client who's site maintenance came up without anyone touching it. What would cause something like this? 

I have theories that it's the backup module. Would that be possible? 

I just went and turned maintenance mode off last night and just now but the maintenance mode eventually goes back up.

I reviewed this thread but it wasn't related: How to access the admin pages for a site stuck in maintenance mode?

Since, I'm pretty sure that clean url's are turned on already.
Drupal will show a site as offline if it cannot connect to the database, so you may want to check that the database is up and running.