Content starts shaking while pulling "pull-hook" (see video)

[Flosef]

Hi,
when I start pulling the pull hook everything starts to shake. In the video I do it very slowly so you can see it better.

This only happens on IOS and when the content part of the page is long enough to scroll! If not, everything is fine.

https://www.dropbox.com/s/y5f3nmu8qvf14aj/this.mov?dl=0

Any idea what causes the pull hook to behave like this?

Thank you in advance!