Ok, so I just spent more time then reasonable researching cow tipping, because it's always fascinated me, also more then reasonable (I'm a girl who loves cows, what can I say) and after finding TONS of websites (Wikipedia, Snopes, The Straight Dope, and more) saying cow tipping is bullshit and impossible (The math doesn't work out with the ways cows are built, cows don't sleep standing up, just sorta doze and wake up easily, they won't just stand there unaware) mixed in with tons of people who say they have witnessed it (But can offer no proof besides "I've seen/done it.") which is pretty much what I've run into in reality, I think I found the most logical explanation for the divide between "Urban legend"/"Done it":
I'm a country girl and I've been around cattle and horses all my life. I just don't buy "cow-tipping." I guess it is the semantecs of the words. Cow-tipping to me would mean sneaking up on a sleeping cow (who is standing) catching them off guard and with a swift push, boom they fall over. This just does not happen. For one thing the cattle I have been around do not sleep standing up.
What I am hearing is drunk teens using brute-force to knock a cow over (which can be done). This is more like Volkswagen-tipping. Enough guys could turn over a Volkswagen and enough guys could wrestle* a cow to the ground. To me that is not a true definition of cow-tipping.