Early in my career I was around some construction operations. It's common practice in big projects like this to pile up as much dirt as you can in a temporary pile on a space that you will be soon be building a large and/or heavy structure. That way the ground can compact as much as possible onto the rocky sub-terrain or bedrock and the heavy building/structure will be easier to construct short term and be more stable long term. (And Anaheim's geology is textbook perfect for this, with good dry topsoil sitting on rocky subsoil sitting on bedrock below)
It's also a handy space to store dirt while you deconstruct other areas that will need grading and the dirt later in the project timeline. It's a win-win for big construction like this.
The space where they are dumping all the earth appears to be the site for the Millenium Falcon ride building, according to the Miceage map. If I were to bet a churro on this... I would say that we will see vertical construction first happen this summer on that Millenium Falcon ride building area. That will be the first area we see structural steel rise up from the cement foundation, which is why the dirt is there first.
The "Expansion Pad" that the last Miceage Update and some insiders are saying will be a Frozen ride is to the east of this dirt mound, or at least partially covered by the dirt.
Curse you Disney!