take your pick from the possible culprits.
- chaos.
- new shader model for the engine with included emissive lighting.
- skylight updates.
- volumetric cloud updates.
- the same bad raytracing stuff for over a year despite Nvidia releasing branches they could pull and include code from right in this very forum
- post process changes / Engine quality switches.
It’s really a free for all. And they don’t really seem to care because “fortnite runs” so…