he knight has an outrageously large plumage on his head. Maybe that's what prevents him from seeing Death attacking him from behind.
The feathers may seem exaggerated with modern eyes, but they are far from unusual. The picture to the left by Hans Wechtlin is from 1501-1526, i.e. a little older than Holbein's dance of death. Wechtlin's knight is no less feathered than Holbein's knight.
Holbein's knight has also been made as a silver plate in the 1600's (picture to the right).
Variations: In Birckmann's version, the knight stands with both feet on the ground and holds the sword with both hands. Valvasor and Deuchar copy Birckmann.