It hadn't occurred to me that that was any sort of "reveal". That's almost always the case in zombie fiction so I had just assumed. If that's even what you're talking about. (In fact in the very first episode, the soldier in the tank reanimated, and there was nothing there to suggest that he was bit or had died of anything but starvation).
Shane's sudden meltdown was quick, but we've seen little inklings of his being insane in the past, and Lori apologizing to him, admitting their affair wasn't as one sided as she'd previously insisted, and admitting that it may be his child, clearly pushed him over the edge. You can see it in his face in that scene. Probably the best acting out of this show so far, really.
Why Rick, who clearly knew something was up, just went along with it for so long, I have no idea. Rick doesn't often make the smartest decisions.