Fortnite has one of the most aggressive update programs in the company, and it is apparent that updating a game so often can cause all sorts of unanticipated problems to develop. Last extended downtime was caused by errors that caused a crippling number of database queries, and Epic went into some fairly extreme detail explaining exactly what was going on. The company has a custom of a certain degree of transparency, so expect lots of updates if this goes on much longer.
It is a big setback for a day that was going pretty well for Epic Games, even if a few server problems over at PlayStation Network have been giving PS4 users a few difficulties. It does not appear to be a terrific day on the technical side of things overall.
Fortnite has had some intermittent technical problems recently, most notably some matchmaking problems that caused the programmer to delay the much-anticipated Playground mode for weekly. Nonetheless, it's been some time as it saw extended unplanned downtime, so we expect that that isn't what we're considering here.
Fortnite has among the most aggressive upgrade schedules in the company, and it is apparent that upgrading a match so frequently can cause all kinds of unanticipated problems to come up.