Painting walls in the house and getting small bubbles

Odd thing occuring. I was painting the walls of our house (new construction, so I was just painting over builder's grade paint) and using Behr interior eggshell paint (no primer in it) and a Wooster roller cover with no problems. I then switched to a Wooster 4" roller for smaller areas in the same room, and the paint started developing tiny bubbles shortly after I'd applied it to the walls. I went back over the paint using the first roller and orienting it vertically, and there were no problems.

The paint was freshly opened, and it hadn't been thinned with anything. The nap was 3/8" on both rollers. Any ideas of why the second roller would do this?