I've had this - it blew me away. Although I tend to agree more with Kelly for the reasons behind it.
With DS, I did not feel safe or secure. I was a little distressed and the sudden urge to push was unbelievable, but more so because my body just forced him out and it was uncontrollable. His head was born in one "push" and his body in the next. I initially thought it was the fact that he was a full 2 pounds lighter than his sister, but Kelly's explanation seems most plausible.

My 2 girls (one before and one after) were born in quiet, safe and controlled environments and I never felt an uncontrollable urge with either. I just pushed when my body told me to - it didn't take over for me.