Wings are a source of weight and lift. Fuselage is a source of weight. Payload is a source of weight. Ignore the horizontal stab and the negative lift in normal flight.
Lift = W_wing + W_fuselage+W_payload. If Lift remains constant after release of payload then W_wing + W_fuselage must both go up, redistributing W_payload between them by acceleration in proportion to their weights.
Since the connection between wing and fuselage also took the full W_payload, then after redistribution some of that weight will be in the W_wing and so the connection between the wing and fuselage will be less. The excess lift will not be able to put all of the weight into accelerating just the fuselage.
Imagine at the extreme a flying wing so W_fuselage = 0. The redistribution from dropping the payload is from wing to wing+; the bending in the middle goes down as the weight is more evenly distributed across the span by acceleration of the entire wing due to the now excess lift.
The connection at the wing root held until the outboard segment twisted up. It looks like the plane was pulling up after the dive to release the powder.
It looks like the forward strut failed; the most common failure mode would be buckling. It's supposed to have mid-strut braces to prevent that, but they may have been damaged previously.
Since the strut was under compression at the time maybe the outer strut fitting failed from a fatigue crack. The one on the fuselage is at such an angle I would not look there first.
See
https://www.federalregister.gov/docu...ng-lift-struts
(3) We are issuing this AD to detect and correct corrosion and cracking on the front and rear wing lift struts and forks, which could cause the wing lift strut to fail. This failure could result in the wing separating from the airplane.