FC maintenance can occur at any time and is usually announced before it takes place, especially if it is scheduled maintenance. Most of the times, EA servers are taken down to fix bugs, glitches and errors, update something important in the FUT database, make a few gameplay changes or due to heavy demand.
One of the most important aspects of Ultimate Team are the EA servers health. The Ultimate Team Servers are the infrastructure that keeps the FUT game mode online. Usually, Xbox is in a different server from the PC and Playstation UT servers.