I've never heard of a reason for Aozaki being there, but the meeting makes perfect sense from a story-telling perspective. The story doesn't allow Shiki to walk around without his glasses all the time, probably to make it more interesting at the time he does take off his glasses. It's like a down-to-earth method of making him 'activate' his ability. In F/sn it's done by introducing magic circuits, in Tsukihime it's done by introducing his glasses. (Note how for example Akiha's abilities are activated in a more classic style, without any specific explanation.)
The need to have Shiki keep on his glasses most of the time leads to the concept of increased stresses on the brain when Shiki does activate his mystic eyes. This, in turn, means that if young Shiki doesn't meet Aozaki, he would very soon die of brain trauma, and the story would end. So the story is in need of a glasses introduction. This is handled by the meeting on the hill. It's a deus ex machina, but a necessary one.