kennertree,
Happens to me whenever I've been out on swaying, nodding, limber stuff for a prolonged period. Doesn't happen if I've spent time mostly solidly positioned on the spar of a big conifer TD. For me, then, that would eliminate the saddle or spur related blood constriction possibilities.
I always considered it a good thing, I figured it meant my brain was rapidly adapting to the demands of variable balancing potentials and my lead-footedness back on the ground was only temporary, lasting 15 minutes, max.
One other consideration, if you wear glasses. I have had to since the fourth grade. I once had a pair of prescription safety glasses with inaccurately ground lenses. Foci of the lenses were wider than the distance between my pupils. Put my depth perception off, made me clumsy for a while everytime I switched between my safeties and my street specs. Also gave me headaches forcing my brain to adapt/re-adapt.
RedlineIt