In zombi movies, as opposed to the Haitian folk-medicine belief/practice, what starts off the rampage?
Satelites coverd with strange radiaton, string, or nothing. Nothing explained, anyway.
I always assumed it was their unstanchable craving for BRAAAAIIIIIINNNNS.
They show up when it’s the time of the season for loving. And if a female tempts you with her charms, tell her no. If you’re careful, you’ll find she’s not there.
Do leash laws not apply to zombies? How about laws requiring you to pick up after them with a plastic bag covered hand?
For some reason I find myself wondering if I’m the only one under 30 who will get this joke…
I’ve always liked the X-Files explaination for zombie behavior:
So, you know, being undead makes them hungry, that’s all.
Killing the parent process with a kill -9.
I get it, Chuck. Even if none of the under 30s will. 
Hey some of us under 30’s like decent music. sheesh.
I know Scully is the skeptic, but that script looked like it was from Season 7, and skeptic or no, after 7 years of running across aliens and the supernatural, how the hell can she say that people don’t come back from the dead? Hasn’t she seen ghosts and things even wierder then that?
They can also be caused by the evil Umbrella Corporation experimenting with dangerous vaccines to create a super-soldier.
Or a bunch of idiotic eco-terrorists trying to free monkies infected with the rage virus.
Damnit, I know you love the animals, but if the animals are infected with horrible dieases, releasing them into the wild is not going to help things at all, retards!
Not to mention the fact the Umbrella Corporation builds crappy safeguards into their bio-weapon labs.
I mean, the computer was a good start, but an air system that takes the virus to the next room? That makes so much sense.
Then there’s the deliberate reanimation of the dead, either for military reasons, or out of scientific curiosity, or out of idealism.
Hell is exothermic and it has exceeded its carrying capacity.
An air tanker carrying Thorazine exploded in midair.
The cost of caffeine has risen past the cost of cocaine and a million geeks all have terrible headaches.
As others have said, it varies with which movie/series of movies you’re talking about.
In the Return of the Living Dead series, for example, it was the chemical “2-4-5 Trioxin” that reanimated the dead into brain-eating ghouls. (Not sure how it made zombies able to infect the living, though.)
I’ve noticed that, in recent years, in movies and whatnot, it’s usually some kind of virus (possibly a bio-engineered one) that raises zombies. Either by reanimating the dead, or by killing and reanimating or simply mutating the living.
Holy Crap! The ways she looks! The color of her hair!
Phil Collins and Ritalin or Worchestershire sauce being used as embaling fluid.
That’s because on TV, skeptics are stupid. That’s why I hate the X-files.
Re the OP:
Sometimes, Death gets retired, and the dead say bugger all to the crowded waiting room.
Sometimes it’s because you didn’t say the magic words correctly.