You don’t even need to put the clamp diagonally. If you cock a bar clamp a little, even with it pretty much running parallel to the edges, you can produce forces that move the piece in or out of square. It’s easy to do accidentally when you are rushing through getting a bunch of clamps onto a complicated piece. I’ve seen Paul cock a clamp a bit to pull a carcass into square. I’ve also seen him go around the clamps on an out of square project and reseat them to make sure none of them were pulling the piece out of square. Clamping next to a joint rather than directly over a joint (sometimes with a caul) can also pull things out of square and can bow the sides of the piece.