The drier and hotter it gets in summer, the more rolling occurs, and the deeper and more compact and oiled the bowl becomes. When it rains, these dust bowls gather water and become temporary watering sites. It's tempting to attribute clever engineering skills to these animals as if they think it through. But this is a great example of an adaptive behavior-environment interaction that likely needs no cognitive thought on the part of the horses. Just one behavior to satisfy one need leads to serving other survival needs.
Grazing occurs in long bouts of two or three hours interrupted by resting bouts.
There does not appear to be an association of rolling with feeding. So I would guess that the fact that your horses roll when they are turned out is coincident with feeding, not related to feeding. A less common, but occasional question about rolling concerns horses which will try to roll under saddle. This is sometimes when they approach a stream, which is just natural behavior. Sometimes it seems related to discomfort from the tack.