Why does cube UV unwrap at an angle? The mesh is square but the UV is skew, at an angle please see images below. I've marked the cube's seams in edit mode, selected all edges and hit unwrap. The UV unwrapped result is in the second image. Why is it at an angle? It would be VERY useful if it kept its 90 degree relationships when unwrapped. (third image shows result when you apply transform before unwrapping - helps with overall proportion but not with the random angle problem.)

Thanks
JK
I've checked out your seams placement and you're right- it doesn't unwrap in the right way (don't know why). To solve it you may snap the cursor (2D Cursor mode) to the very bottom vertex with Shift+S-->Cursor to Selected, select the nearest one and rotate them to create a straight line (you may then scale the vertices by Y axis- S,Y,0 to make sure the edge in a perfect horizontal position). Then pin the vertices with P. Finally unwrap the mesh again (U-->Unwrap) and get rid of pinning (Alt+P).


You may also try to place  the seams in a different way e.g. as pictured below. As you can see it unwraps well now.