Fps drops in just one game?

I usually only play Dead by Daylight and Fortnite on my PC (other games in between but not active). Everything went well in Dead by Daylight, constant 60Fps (highest frame rate in the game). Then in Fortnite (where I usually have between 100 and 200 fps) I often had fps drops that went below 20 fps. What can be the reason and what can I do about it?

Which settings?

Everything low viewing distance epic and 100% resolution on 1920x1080

I believe your PC is fighting Fortnite.

Very helpful haha