Vorticular turbulence?
The shuttle does rotate (pitch I think)rather regularly to keep a certain orientation with respect to its Earthbound nadir. The think is, it's not a smooth event judging from the video I've seen of the crewmembers on the middeck appearing to suddenly be propelled in unison towards the same bulkhead.
More likely, the spores become segregated in eddies or convective boundaries in the water they're suspended in.