Sunday night, I could be doing so many things other than breaking Jack's toys couldn't I?
Well, I am that boring sometimes. Sorry Jack.
So, the combo that caused the weirdness is CustomBounce + percentage base movement + transformOrigin in a shape element of an SVG.
As you will see in the pen (at least in Chrome), the element gets moved to the 0,0 position according to the transformOrigin you define. Not something desirable if you're trying to squash an element.