I know the secret
this drove me nuts until I figured out what it was doing one day, and then at least it was a lot less confusing and annoying as to why it ended up where it did sometimes.
In previous versions of PSE would drop the file you are dragging into the document on top of the layer that was selected in the layer palette, no matter how or where you dragged the new element on there. Starting with PSE2015, they changed it so that it now drops it above the layer that your pointer is over in the document when you release the button. So, you have to make sure visually that your pointer is on top of the layer you want it above when you 'drop' it. If the layer is a deep one and mostly buried by other things this can be harder to do.
Oh, and it always drops the element in the center of the document now too.