Well a dutch researcher called Peter Aerts actually suggested a different reason. Perhaps, he suggested, lizards were not trying to run bipedally on purpose, but rather they were trying to do something else, maybe they were trying to become more manoeuvrable. One way to become more manoeuvrable, is to shift all your mass backwards (which makes it easier to turn corners), and then accelerate quickly. Unfortunately for the lizards these things have a side effect, just like when a motorcycle accelerates too quickly, when a lizard shifts its mass backwards and accelerates too quickly, it can cause the front of the body to pop up, like its popping a wheelie. Seen in this light, bipedalism in some lizards might have been an accident, just a consequence of accelerating too hard, and this seems to match some of the data I have collected before on lizards. There is certainly an acceleration threshold where a lizard will pop on its two back legs, and a model produced by Peter Aerts even predicts when this should happen. And this model matches the data, for most lizards.