Lizards are cool. (along with snakes, spiders, monkeys, dolphins, … damn, almost all animals)
Take a look at The Scientist for a nice video of lizard “wheelies” and some text explaining possible rationalisations for it.
Leading explanation seems to be torque based:
“There’s this one acceleration where once the lizard hits it, it really has no choice but to go bipedal because the torque starts to move its head up” – Christofer Clemente
Previously it was thought the lizard might be going faster because of running on two legs. Now it’s a case of the lizard running on two legs because its going faster.
But it’s not so simple, and the mystery is not yet solved. Lizards appear to have some control over
their wheelie pops. Usually electing to go up before hitting the threshold estimated to physically force them up. Also it may still be an advantage as far as speed over rugged terrain.
Clemente also mentions visibility – though I’m not sure how this is a survival trait. Unless it’s meant it a feline-like defensive sense, that the lizard is making itself look bigger and meaner (while it runs away?).
One thing I don’t see mentioned is climbing. Goannas, and I’m guessing other lizards, often dart off to the nearest tree when running for cover. One book I have warns that undiscerning lizards can confuse standing persons for trees, leading to a not-nice scenario for man and beast. Could bipedalism give some sort of boost to jumping up a tree?