Fortnite.
The unimaginable runaway success of Fortnite took Epic from "market entrant" status to "dominant industry leader". It allowed them to pour hundreds of millions of dollars into Unreal development, and enabled them to finally release the engine under a freemium model to indie devs. That, (combined with the iOS ad-pocalypse) has led Unity to a long slow decline in developer share ever since.
The Unreal engine has been around for over twenty years now and has been considered one of the top mainstream engines for over a decade. I'd hardly call that a "market entrant".
As another commenter pointed out, Unreal went freemium two years before Fortnite anyway.
The whole Unreal engine with the editor and such is powerful toolset and you can tell it's an engine geared toward professional game designers and artists, basically whole studios, rather than programmers. Unity is really fun to play with if you are a single indie dev. Also it's much better for mobile development. You can do all with Unreal engine too but it's "meant to be" for FPS/TPS games and anything other than that is not harder just you need to "fight" the engine sometimes.
And the Unreal engine is open source. Well it's like open access because there are some very strict rules and limitations around it. But at least it's good for personal use https://www.unrealengine.com/en-US/ue-on-github
The downside of course is that it's not a master of anything, and Unreal has great showcases to demonstrate that.
Unity isn't going away, devs aren't going to magically switch to Unreal for the above reasons. Additionally the transition is massively disruptive for a business built around Unity. You don't just magically become Unreal experts as a team by reading a few tutorials and building a couple of prototypes. Actually shipping a game cross-platform requires a huge amount of technical experience to optimize and work around issues that each brings.
Unity is arguably getting worse and seems to lack a cohesive strategy. Last I checked, which was a few years ago, the engine doesn't have functioning networking feature (online multiplayer). They used to, but they deprecated it before failing to launch their new one. There's other stories in a similar vein.
You can of course make it work with third party plugins but it's not a good sign of engineering quality.