sideways rotation

Shyanna Busch

I thought I saw a post about this awhile back, but I can't find it.

Today, I was at work and storm clouds came in. I was looking out the front door, and I saw this cinnamon bun looking cloud a little ways in front of me. It was spinning but the spin was vertical, not horizontal. It was really neat to watch and I was kicking myself in the butt for not bringing my camera to work, despite the "feeling" that I should have grabbed it before leaving the house. I missed out on a double rainbow shot too. grr

Anyways, has anyone ever seen or heard of this and what causes the "sideways" rotation?
I suppose what you saw could have been a roll cloud — which rotates horizontally like that I think because of wind shear between the outflow (which it's on the boundary on) and inflow, or just the forward momentum of the storm.