Here is a different video that shows similar behavior:

Cat harassed by two magpies

I agree that the magpies are not just having fun but doing something purposeful. They probably recognize the cat as a dangerous predator and they are trying to drive it out of their territory. Maybe there is a nest or young nearby that they are trying to protect, or maybe it is just their instinct to drive predators away whenever they can.

If you look at "related videos" there are several like this.

The magpies (or crows) always have the same approach: they try to sneak up behind the cat and peck at its tail. And sometimes the cat seems to be trying to use its tail to lure the magpie within striking distance.

In some of the videos, the magpies are chattering loudly. I would take that to be a warning call--warning any unwary members of their own species to beware of the cat. That's what I would expect. In the original video, were the crows being quiet, or did the video simply lack a soundtrack?