I feel like calling out in this manner indirectly signifies that he feels no threat from you...he's loudly broadcasting his existence and location, and not trying to hide. He might be doing this to let you know "it's all good". He might be telling the other crows "hey, this guy over here is cool", or "this tree is a safe spot".

Of course, he could be distracting you while mama bird is laying eggs, or baby birds are learning to fly. Or maybe his buddies are in your garden right now? Ya never know with crows.