This is a very important question - whether Bowie was on a leash. If a leash was still attached to him, it doesn't mean she didn't drop it and allow him to run ahead, knowing that he would come back when called. I cannot see the killer attacking Katie with Bowie attached to a leash attached to her hand. He would have no way of knowing how that dog would react. If the dog was up ahead, he could lure him over and quickly neutralize him before besetting Katie.
I, too, go round and round about whether this was a random act. I cannot wrap my head around an opportunistic killer attacking someone with a larger dog. But of course I'm not ruling it out. If he were someone who had a beef with Katie, he had to be following her, all prepped with a kill kit, in the off chance she stepped into the park and he could make a move... it seems a stretch.
MOO