From the look of things, Id say your beans werent in motion enough at the start of the roast.
The usual advice is to ensure your popper has a chimney, i.e. a tin can shoe-horned down its spout to prevent runaways, and to tip it on a slight angle and/or stir the beans during the first part of the roast until they get a bit of motion going on their own.
It looks to me like the beans on the bottom were burnt.
Ive only done a few popper roasts so far, but my results have been quite even.