Buzzard v Crow (1 Viewer)

I watched a buzzard take off from its local perch (football goal posts) to day (february 15th). As it start to circle and gain height, it was tracked and then repeatedly buzzed by a crow. The buzzard did not seem to react but moved off after a few minutes and the crow decended back to the tree tops from whence it had came.

Why would the crow carry out this course of action against a larger bird.

It's a common way of reacting to a potential threat or competitor - harrass them long enough and they'll go away. Crows do it all the time. Buzzards and other raptors also do the same with larger raptors, for example I've seen a Kestrel mobbing a Peregrine once, and two Yellow-legged Gulls drew my attention to what I first thought was a juvenile YLGull - turned out to be an Osprey. Some birds of prey, for example eagles, will turn in mid-air and slash at the attacker, which sometimes results in a case of "suicide by cop".
For a birder, the commotion this mobbing creates is a good way to find predatory birds - especially when you hear birds calling which are normally rather silent, as in the case of the aforementioned gulls (or the Peregrine, which sounded genuinely annoyed).