I just had it made to a print 16x20 with no issues, 

I beg to differ, one issue that stands out is this:

It’s not very sharp, is it?

So, that is why you got the quality rejection reason. Your 16x20 print might be ok for you, but not for a lot of other people!

The shutter speed was too slow, resulting in blur.

Please use a faster shutter speed when taking the photo.

With a close-up shot like this, sharply focussed details matter. 

I’m seeing motion blur on the riding crop and rear saddle. 

Hi, nice pic.

Speaking as someone who doesn't know anything about equestrian sports, I get the impression that the area around the hands feels a bit busy, as if there's too much visual information. It's just a personal impression, though.

It might be worth creating a cleaner, more minimalist version of the image, with the reins arranged a little differently and/or without the whip.

Shadows need to be opened up.