In my opinion, using aircraft as "attention getters" during social events is a poor idea, and to be avoided. For obvious reasons, a failing during such "displays" ruins the event, and probably lives. Pilots should consider declining such requests, and just stand there with everyone else and enjoy a finger food.
As silly as this pilot was, I have to hope what whatever he was dispensing over a crowd had more colour than mass. Though I agree with the theme that a sudden load release changes the physics of the flight, I would be surprised to hear that this load, in this situation, was a major factor in the structural failure.
The video shows the left wingtip twisting upward (increasing angle of incidence relative to the root), then the entire left wing folding up from the fuselage. It happened so fast, that the additional momentary lift of the left wing could not induce any brief roll to the right. So I doubt that the wing spar itself failed, but more a failure of the wing structure in torsion, so that a sudden increase in lift resulting from the torsional failure, caused the left upper wing strut attach at the fuselage to fail. As those wing struts act in compression, the failure would be instantly total. I opine that overly exuberant pilot pushed a less than ideally airworthy airplane just a little too far.