I'd venture a wild guess that the revenue split business model behind Unreal Engine, and the strategy around spending tons of money to bring people over to the Epic Games Store, are not sustainable. They probably have been generally subsidized by the huge amount of cash that Fortnite has brought in.
Maybe the Fortnite well has dried up? 2023 has been a strong year for news game releases, and it's possible that Fortnite has lost some of players' attention?
Hard to say, but it's looking like Unity are not the only ones struggling to keep their business afloat.