Bruising is caused by bursting of the blood vessels under the skin, and this can happen to the palm of the hand in the same manner as elsewhere on the body. It happened to me in my job a few weeks ago and was bleeding painful!
Seriously? Sorry to hear about that. How'd you get that bruising anyways? I mean, I can't imagine someone slapping something/someone hard and end up with their palm bruised. Heck, I have never seen a bruised palm.