Why did my cake go Offsided?
You might want to include your recipe for the buttercream. That could help people figure out one of the reasons for your cake layers sliding around.

My thoughts from looking at your luscious cake photo are:


3 layers is a pretty tall cake! 2 layers would be more likely to stay stable. If you want 3, you could make them thinner, or even cut one of them in half. As long as you don't have a sliced edge at the very top, it doesn't matter that frosting will kick up crumbs from the sliced edges.
Layers don't look quite level (i.e. flat). You mentioned leveling the top layer. Maybe more important to level the bottom layers, if you don't want the upper stories sliding around.
Is buttercream a little too liquid and slippery? Knowing your recipe might help people determine this.
Possibly a little tooo much buttercream in between layers. I don't mean too much to be delectable; I mean too much for mechanical stability. It's hard to tell how much is in between the layers, but it does look like a pretty thick coating on the outside, so I'm guessing you were also generous in-between.


All that said, this cake looks terrific, and very rich!