The Great Egret landed too close to the Heron nest.
All the birds are still working out their territory so this will be constant until lines are drawn.
An Egret rarely confronts an angry Great Blue. The Heron is larger, but the Egret more aggressive. Only during nesting does this happen.
After watching this for several years you can spot the different styles of these predators. The Heron is low, always waiting for the other to get closer. The Egret almost always hovers higher up when fighting. I think that evens out the size difference.
This time the Great Egret stood his ground for a short time, then dropped off the tree top. I have seen Egrets stand ground and make the Heron back off in the last day.
Territory is being established and until the chicks are older (and a target) they will avoid each others nests.