I can't help feeling that some damage has occurred to the meristem (growing point) which sits deep inside the crown. The fronds may have just expanded using any residual sugars and liquid from their bases. You will have to wait and see I'm afraid. Not really anything you can do except the normal procedure of keeping the trunk sprayed and moist.

As Fractal says, humidity (particularly of the trunk) rather than feeding is the key. I would seriously doubt that Miracle Gro would do a Dicksonia too much good....it's formulated with a heavy emphasis on Nitrogen & Phosphorus to boost flowering and rapid growth. Ferns in general have very low nutrient requirements and can easily be damaged, burned or even killed by over-fertilisation.