The reason for this, is their control algorithm. It uses a inverse kinematic to compute the desired motor commands in the joints. When the legs are stretched out, it is in a singular position where the resulting motor gains would be infinite high.
Reminds me of a quote from the always incisive Karl Pilkington (to keep this family friendly, google "asimo karl pilkington").
"It walks like it shit itself" -Karl Pilkington
HN isn't for kids, i don't think the family filter is required.