Having guided out of Hermiston on the Columbia for 6 years I have seen a lot of jumping Sturgeon (not just hooked ones).
Like AuntyM says, there are many theories out there, but the one I have most resigned myself to, after these years of watching/catching them, is that it mostly but not always happens in hard current. When a big fish is worn out and they empty their air bladder and sink (you old Sturgy fisherman know what I'm talking about), the only way to get them off the bottom was to get their head up and then the current would force them to the top. I think they cruise around in the hard current and occasionally get their nose up while they are swimming along at a good clip and the current causes them to shoot to the surface. Maybe when the light starts getting brighter, they swim faster, and the next thing they bust the surface. I know this cannot account for all the jumps, but I think it surely accounts for many of them.

MC :p