Essentially, there are a whole load of different zombie 'types,' all which put forth their own reasons for the rise.
I'm not really a big fan of 'no matter how you die, if you kick the bucket you come back a zombie' zombies. Only time I've really liked this is in Maberry's ROT AND RUIN.
I'm more into the 'contaigen' model of zombies, where zombification is more or less an illness which transforms you into a zombie, much like the walkers in The Walking Dead. (or Geeks, for those that prefer the term.)
An interesting point, and one I've thought of many a time - if zombies are after flesh, how do any make it to the point where they have one bite and BOOM - zombie? Wouldn't there just be a bunch of eviscerated crawlers twitching on the ground, risen but unable to move because of massive muscle-tissue damage? Well the idea isn't just that the still mobile 1-bite victim managed to get away, but because of one of two concepts.
One, the virus that transforms the person into a zombie realizes that to best transmit itself, it needs to create spore colonies (read: zombies) capable of infecting others. This would rule out tearing victims to shreds. While the idea that a virus can be self-aware is a little out there, stranger things in evolution have happened. Just look at the way trees disperse seeds - trees have 'realized' (I realize this is a HEAVY dumbing down of Darwinism and the like, but bear with me) that animals are willing to eat fruit, and therefore will encase their seeds in edible material so that their seeds can grow after they are excreted by the animal, or - if you really want to look at it - host. A cold virus makes you cough, spreading the virus further, rather than killing you outright (also a byproduct of immune system, but hear me out).
Secondly, the idea that zombies are attracted to non-infected beings rather than infected beings. It wouldn't be too hard to stipulate that after biting a living person, the zombie reacts to the newly-infected person, seeing their friend trying to get away as a non-infected, and therefore more desirable, meal.
I dunno - I'm not a big fan of zombies that HAVE to eat flesh, I see it more as a desire to bite and infect. I mean, if they were looking to kill, the human body is more than armed with better weapons (like hands and feet) to kill, rather than teeth.
Anyway, every zombie novel more or less creates it's own rules - the zombies in my novel GREY DOGS aren't like others, just like Day by Day Armageddon zombies aren't like Romero zombies, etc.
Hope it helps. That devolved into a rant pretty quick, didn't it?