Probably doesn't help when the original release of Witcher 3 was a broken mess at times on Steam. More people playing shows that CDPR learned from the previous issues. Also, there's a new DLC releasing soon, people are probably getting ready for it.
Witcher 3 hitting higher steam concurrents today than they did at their original launch is gonna push more developers to release more stuff for their older, already wildly successful games, and y'know what good