FL/PE2 Question

When doing FL/PE2 I find my body moves forward (in the direction my feet are facing) as I rock. I'll start at the top of my yoga mat and by the end of 60 reps I'm at the bottom or sometimes even almost off the mat entirely. Did anyone else have this problem? Clearly I'm doing something wrong. 

Sometimes I move a little as well. I read a coach on here say that it will go away as you get better--I imagine it stems from uneven rocking. 

I twisted round to the side at first but after a while it stopped.

Thanks Julia and John! Good to know 

I imagine it is a caterpillar effect from breaking hollow.