I don't know why this happens to me sometimes. I crumbcoat my cake w/ the same color it'll be iced in. When I frost the cake with the final coat it becomes very marbelized looking. Like the frosting isn't mixed right, but looking at the frosting before it's applied it looks mixed really well. I spend alot of time making sure that it's mixed right.

I do use the hot water/spatula smoothing technique, so I wonder if that has something to do with it?

As the icing sets, it can change a bit as the sugar continues to dissolve. One you finish the crumb coat, you can remix the icing, on very low, to try to prevent that look.