FL/PE2 FormC + Problem - I really need a coach's eye on this

Lately I noticed that my body is scooting forward a bit and sometimes....A LOT... during the rocks.

For some reason I start and end FL/PE2 in different locations.

Could this be happening because I crunched too strong and too high with the abs or is there something else I was doing wrong? What is the solution for this bad habit?

 

Also, please check this video out because I think that's my best attempt of 36r. I'd be glad to receive some feedback and advice on my form :

Thank you!!

 

 

This happened to me at first too, I noticed it more with the straddle rocks; it seemed to just disappear after a while without effort to change it.