Watch it again from the camera behind the goal. Bachman watched the pass, the run onto the ball, and then did not react to the shot. International goalkeeper, he will tell you.
Think this quote on BBC site says it all. "West Ham have looked very comfortable since turning this game their way. They look far more likely to extend their lead than Watford do of finding an equalise" And they were fully on top before . Midfield over powered and they are really stretching us up front.
Looks to me like he reacted as though he anticipated the shot going the other way - bit like going the wrong way when facing a penalty.
Having watched it several times, from the moment it left the player's foot he watched it with a clear open view go past his left-hand side, making no reflex action at all.