So there are plenty of reasons to want Bill O'Brien fired. But 2 that stand out from the last 3 days for me:
1) The video of O'Brien going off on a fan at halftime of the Denver game, which was just released last Friday. Anyone who can get that angry at something that he must deal with on a regular basis, to me, is mentally unbalanced.
2) O'Brien having to call timeouts twice in yesterday's game on 4th down because he didn't know what to do. The first one, he took too long getting a play in on fourth and one from the KC 13. Whether this was because he wasn't sure whether they got the first down or not, or he took too long deciding on a fourth down play, both are inexcusable for a HC in his sixth year. As a result he burned a TO (which might have helped the Texans get an easier FG attempt at the end of the first half) and then decided to go conservative and kick a FG. The second one, it was fourth and 4 with 11 minutes left in the game and the Texans down by 17 on the KC 42. O'Brien, in explicitly sends the punt team on the field, and then after Deshaun Watson tells him they should go for it, he changes his mind, but has to call timeout because they wouldn't be able to get the offense back on the field in time.
My biggest criticism of OC's who become head coaches is the ones that can't get their heads out of the Denny's menus and have to keep calling plays. It hurts their ability to adequately manage the whole team during a game, and they become less effective as OC's because they are distracted by other head coaching duties. O'Brien is a prime example of this.