A quick video about what a death wobble looks like.
Death wobbles come from a strong axle with low load rate springs. Typically the springs are strong enough to twist the axle when the tire hits a bump in the road, but does not have enough strength to dampen the torsional movement of the axle. This results in the king pin angles moving violently in opposite directions. Typically the vehicle must be brought to a complete stop for the death wobble to subside.