Perhaps a culmination of things? This is not the first time Reid has had problems with his boys (2 of them). Drug abuse/addiction, accident under the influence, drug dealing, and pulling a gun on someone, to name a few. It is not my place to judge anyone, but family problems can eventually wear on someone and cause lots of conflict. I think many people would have chose to walk away from coaching and get their house in order, Andy did not, for whatever reason.
Possibly , as it appears, good defenses have a blue print to beating the Chiefs; Keep Mahomes contained, get pressure so he can only work short to medium routes and go with a cover 2 approach not giving them the top.
Finally, the Chiefs defense is just not good enough so the Chiefs have to score 30 plus a game.