FIFA 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 FIFA Ultimate Team are the EA servers health. The FIFA Ultimate Team Servers are the infrastructure that keeps the FUT game mode online. Usually, FUT Xbox is in a different server from the PC and Playstation FUT servers.