It is a design fault. A-ha! A bit like us humans even though, unlike us, theirs is a facultative bipedalism. We pay for being bi-peds with bad backs and more than normally painful birthing process, among other things.
For years scientists have pondered why dragon lizards use just two of their four legs as they sprint across the Australian outback, and after putting the creatures through their paces on a treadmill they have the answer – it is down to a design fault. The reptiles usually walk about on all fours but will rise up onto their hind legs when they break into a high-speed run, taking on the appearance of a mini dinosaur.
An extensive experiment, which saw 16 species of dragon lizard sprint on a treadmill, concluded that the creatures run on their back legs because the rate of acceleration alters their centre of gravity, making it impossible to keep all four feet on the ground - the reptilian equivalent of a cyclist's wheelie.
Dr Christofer Clemente, of the University of Cambridge, said that rising up on hind legs was energetically costly, but the price that lizards had to pay for the speed and manoeuvrability that turned them into "the jet fighters of the Outback". (Link)