Playing some poses causes the female sim's both legs to go up and down

[DeadlyShadow]

Could someone help me with this? When i play some poses the female sim's both legs go up and down. What is causing this, the heels, some type of shoes, sliders or cause my sim is quite thick? This seems to happen only on a few of my female sims, never the male. Also i know it's not from the pose.