Three facts about this Cow Video: 1) Cows are not supposed to be constantly/ forcibly impregnated to produce milk. 2) Cow's baby calves are stolen from the mother cow in order to produce milk. 3) Cows do not like having their calves stolen, it is the same for any mother having a baby stolen.
The cow who attacks the dairy farmer in this video is NOT the mother; she is one of the cows who had all of her babies stolen by the dairy farmer in the past. The mother cow, who just gave birth, is most likely sore and in pain, shows up at 0:08 to help her friend that attacked the dairy farmer (notice the umbilical cord trailing from her vulva). A third cow, who didn't see the baby being stolen, rushes to help the two cows upon hearing their distress calls.