I'm going to assume that this beveled cube was made from a Cube mesh.
When you add a Cube mesh to your scene, it automatically creates a UV map for you. Beveling the cube without unwrapping it after does not change this map by much (although it does result in some weird mapping).
However, when you made a seam and unwrapped the mesh, the old UV map was trashed. Blender now tries to figure out how to unwrap the whole mesh to a 2D space with the seams provided. However, Blender cannot do this because the provided seams are not sufficient.
Now, imagine your mesh was made of paper. You want to cut the paper at the appropriate edges so you can "unwrap" the paper to make it completely flat without tearing it. If you only cut the edges you marked as a seam, you cannot do this.
This is what Blender is doing. Because it cannot completely flatten the mesh, it gets confused and makes a whole mess of overlapping faces.
When marking seams, use the strategy above to try and place seams at the correct spots.