This is called "mobbing" behaviour and is often done by groups of crows, ravens and other bird species such as gulls, terns and jaegers at the breeding colony, Eastern Kingbirds (NA Tyrannid flycatcher) etc. It is known from experiments done many years ago that the shape of a hawk-type bird elicits mobbing behaviour, whether the hawk in question poses an actual threat or not. The evolutionary "rule of thumb" is mob anything that looks like a hawk. Mobbing evolved because overall it was beneficial to mob because often a real threat from a predatory bird would be averted. In this case the Red-tailed Hawk poses no/limited threat to the crow/raven so the interaction is one-way, from the corvid to the hawk. Mobbers are very good at staying out of the way of hawks and other species that are frequently mobbed such as herons. They general fly behind the mobbed bird and harass by diving at the bird and making lots of noise, even special mobbing calls. When this is done as a group, it is very effective at deterring potential predators. I suppose the hawk could turn around and clip the crow around the ear as they say, but in most cases it's just better to flee.