We can only speculate as to what is wrong, but it surely doesn't help that Merlin doesn't give the budget for spare parts to be ordered in advance and stored at the parks.
I remember in 2013 when Oblivion at Alton Towers was shut for several weeks due to its lift hill chain box needing replacement. They didn't have the budget to pre-order spare parts so had to wait weeks before a new one was delivered from B&M. The exact same thing happened to Galactica last year and that was also shut for a very long time. Meanwhile SheiKra at Busch Gardens has not one but two spare chain boxes ready in case there is an issue with the current one. (They only take a couple of days to install and get going.)
We don't know what has happened with The Swarm, or even if it is true that it will be closed for the entireity of the park's busiest period of the year, but I just hope it isn't Merlin's budget constraints on spare parts that have lead to extended downtime yet again.