These zombies are the creation of the mad vampire necromancer Strahd von Zarovich. They are the end result of a necromantic plague which is propagated both through contact with these zombies, as well through the influence of the Blaspheme Doru, undead son of Danovich,the village priest.
Strahd is using Doru as a focus through which he can spread the taint of this undead plague. If Doru is destroyed, the plague will come to an end with him. However, while Doru can think of nothing but turning all those around him into infected parodies of their past lives, Strahd tries his utmost to protect his investment.
The reason that Strahd is so protective of his creations is that this plagues has mutated several of these undead into even more resilient and useful creatures. These creatures include what the villagers of Barovia refer to as the Boomer, the Hunter, the Smoker, and the Tank.