What you saw was negative G induced by the pilot who applied forward yoke when the plane surged upward.
You are mistaken, there is no negative G at all and there's plenty of evidence it's all positive G.
First, there is no way on God's green Earth that a pilot will push the nose over that hard, that close to the ground. Second, there's no reason to. Who cares if you balloon after you drop? Third, ballooning after dropping a load is pretty darn minor, and certainly not a reason to stuff the stick forward. What are you going to run into if you balloon? Air. What will you hit if you stuff the nose down? Rock. Air beats rock. .
All that stuff aside, look at the scenario. In every video I've ever seen of this the water bomber is either descending over a ridge line before the drop, climbing over a ridgeline after the drop or both. He does this because fires happen in very inconvienent places and he has to be very low over the fire otherwise the water spreads too much and does little good. In all of these scenarios, his primary concern is to not hit the ground. In some circumstances he may use some pressure on the stick to keep the nose from pitching up but most likely he's pulling.
Last. Again, in every video I've seen the wings fold up, not down. Proof positive it's positive G. If you have one showing the wings folding down I'd love to see it. As I said before, the only time a pilot would intentionally come remotely near his negative G limit is if he's avoiding a midair or in some extreme aerobatics.
