My problem even before this story line began to emerge was this: if being bitten/zombie blood makes you turn, then what about all the blood that has gotten all over these people as they have hacked zombies to death? They have been slavered on, splattered head to toe, covered in guts, etc. NO WAY has zombie blood not crossed a membrane barrier -the mouth, nose, eyes. Yet no one had, before now, turned without a bite.
So now that's happening. Why?
My theory is blowback. The way Jarhead and I worked it out, if you are bitten, you get the virus in your system via the bloodstream in vast quantities, and an infection ensues. You die of the infection and then reanimate.
But if you get a secondary contamination - for example, blood splashed in your eyes or mouth as you slam a hatchet down on a dead cranium - then your body is able to fight it off in small quantities. But when you die, the body's defense mechanism no longer protects you, and the virus takes over. Then BRAIAAAAIIIINS...
Both Shane and Randall had been up close and personal to zombies many times. Neither of them had died of a head wound. The two dudes in the bar won't reanimate because Rick shot them both in the head. Otherwise, they probably would have. Same thing goes for Dale.
Jarhead remembers in the comic (and I do, vaguely) that Rick goes back to where they buried Shane and smashes his head in to prevent him from coming back. The TV show only slightly followed the comic for this story line. In the comic, Shane was attacking/raping Lori and Carl actually shoots and kills Shane outright to save his mom. He had not yet reanimated when Rick goes back to smash him, so that is definitely a deviation.
Kind of reminds me of The Stand. Killer flu, some are immune, some aren't.
In one of the book series that Jarhead, Sani and I read (Monster Hunter International), all members of the Monster Hunter teams are decapitated prior to burial. Most have been bitten by or exposed to vampires during their careers, and they would reanimate as a vamp after they died unless they are headless. The virus stays dormant until death. That's where I got my theory.
In the MHI books, a person killed by a zombie also reanimates immediately. SO it could happen.