For Homo Sapiens (yes, that's us, human), it is easy to open a door. But for humanoid robot, opening a door is difficult. The robot should balance itself on two feet and give enough swing (or energy) to open a door. Swinging means unbalance position. Then a robot should be back into stable or balance position, which calles standing.
IEEE Spectrum published an article about how a humanoid robot opens a door.