Morning Glory clouds are a specific and more unusual type of roll clouds, which are more common, and not usually as long.
Roll clouds typically occur in the lower atmosphere ahead of a storm front. Warm updrafts in the storm front push cold air up, which then flows down along the sides of the updraft. The cold downdraft then bounces back up a bit setting up a wave-like structure in front of the storm.
On the upswing, the cold air forms a cloud. Evaporation of the cloud causes a downdraft on the edges that erodes the cloud, forming a roll. If the wave continues, a series of roll clouds, called a street, can form.