I have seen this behavior before. It’s even more common with the Great Egrets.
When in an attack or defensive position the birds will almost make a show out of sharpening their beaks.
It’s probably a form of intimidation.
Egrets have been moving in on this Great Blues nest and she has tiny young. It will take more than a single Egret to drive her away. Both Herons and Egrets will stand their ground.
Above you can see a small ‘tuft’ of feathers below the Heron.