Why is there a spiral in bongs?

The spiral shape you are observing is a result of a vortex current which has formed as a result of the pressure difference inside the device and the relatively higher air pressure outside the device. The area closest to the air opening will have a higher atmospheric pressure than area further away from the opening resulting in the rising spiral you see. It is a very similar phenomena to the spiral you also see when you flush a toilet, the only difference is instead of the crap going into a sewer it goes into your lungs.