That looks like compression artifacts to me. That area is in the foreground with a lot of detail is moving fast. Sort of like when capturing a scene with confetti pouring down, it reeks havoc on codecs.. Don't know if it's actually a YT codec issue, or the original, but I think either way it's compression issue.
My guess would be YT, since they compress the hell out of things to save server space.
I seem to remember that there's a guide somewhere on how to compress video best for YT to get the most out of your video when posting specifically to YT, I just don't remember where I saw that and what's involved.
I use Vimeo, which in my opinion is much better about things like that.