Crazy!
My original thoughts were:
Wooden bow? How much string follow when unstrung?
It almost looked like the limbs sent the string with a lot of initial speed but had a hard time to catch up with the string. No matter the stretch, no string will stretch that much. Just measure the loops in the picture. :0
But I think CatSplat nailed it:
Camera lens distorsion sounds like the most plausible explanation. Try filming the same shot (bow/arrow/string setup) with a regular video camera. No need for slowmo since the effect was noticeable without slowing the film. Also, if the string did that and if we can see it that clearly without slowing the film down, shouldn't we be able to observe the same effect directly?
Edit: ooops, I just saw it. It's a schultz tembo. So string follow is out of the question.