Why is a Deadman called so? Where does the name come from?
imsodin's explanation (it's a heavy object you bury in the ground, like a dead body) seems very plausible. Just to add some history to it, here's the earliest use of the term listed in the Oxford English Dictionary:
a1852 W. T. Spurdens Forby's Vocab. E. Anglia (1858) III. 12 Deadman, a piece of timber buried in the earth, to secure posts, or other timbers by.
The next recorded usage is:
1901 Daily Colonist (Victoria, Brit. Columbia) 15 Oct. 5/4 A deck hand..was killed by being struck on the head by a ‘dead man’, which is a post imbedded on a [river gravel] bar to haul the steamer over.
So the term has existed in agricultural and nautical contexts for some time (and when you're burying something in earth rather than snow, the connection with burying a body is a little more obvious).
The DMM device you linked to is as far as I know the only product that has this name officially, but a dead man anchor is a general term for what is in mountaineering also called a t-slot anchor. In that case you simply dig a trench at right angle to the direction of pull, bury an object and attach a rope that holds the weight. I presume that as the DMM device is applied in similar circumstances (it is more dependent on very compact snow but faster) it has the same name.
There is an explanation for the name itself, but beware of obvious morbidity:
You dig a hole and bury an object in it, which is quite similar to a burial (while you do not usually attach a rope to a coffin). Moreover dead man anchors are often used as anchors to fixate a free standing object (like a mast). The line between the anchor and the object is called a guy-line, sometimes simply called guy. I do not know which was first (guy or dead man), but I can see how the idea of "attaching a guy to a dead man" can come up...