For me, it's an overly-strong handshake (especially in a business setting). I think a firm handshake is the way to go, but there's firm and there's "Ahaha! I could crush your hand if I wanted to!".
It just reeks of inadequacy issues to me, and I find it hard to believe the person is doing it without knowledge. Don't squeeze my hand until it hurts, it's not a pain-causing contest. No one thinks you're more powerful because of it, you just look like a little person who is desperate to one-up people.