Can someone explain wtf actually happened to Cooper Kupp?

So he gets hurt week 3. Okay, fine. When he comes back he's doing excellent. Volume, consistency, has upside every week. He probably averages close to 20 ppr points over the 7 game stretch after returning. And then out of the blue with literally no warning he's completely cooked the last 3 games of the season plus the wild card game. 4 total catches over those 4 games. This is a veteran with one of the best WR-QB relationships in the game. Just completely shats the bed. Isn't even getting targeted. Did something happen schematically? Did he kill Staffords dog? It's just really baffling to me - the most plausible explanation is that he must playing through some injury but there's no reporting on that.