The creature is an alien resembling a reptile. It is around 1m tall. It has scales and a tetrapod-like skeleton, but lacks a tail. It breathes through paired siphons in its neck. It has no nostrils or ears, but instead has many antennae along the face, rostral to the eyes. It has cheeks, which support antennae, and a small mouth like that of a human. It has forward-facing eyes, similar to carnivorous mammals. It has wings like a kuehneosaurid, but it is flightless. What evolutionary pressures could lead this creature to evolve an orthograde posture, or at least a similar stance?
It’s efficient, and they like trees. Much the same as us.
Swinging your limbs for balance is easy to do if you’re bipedal. Without a tail the only limbs you can realistically use are arms/wings. If you climb trees you naturally use your limbs in opposing motion (even if some of those limbs are wings), so using opposing limbs to maintain balance is really not that much of a stretch. Perhaps your creature evolved from a glider that climbed trees and then leapt onto prey (explaining the vision, the lack of flight and the wings all in one) then moved to bipedalism later as having a naturally elevated vantage point became more useful.
In all honesty I’m not sure a trait this innocuous needs a strong evolutionary reason to exist. It could simply be a matter of chance and author fiat and nobody would complain.