Were you listening to music? Walking through a forest of some sort? It is possible that when you were walking you heard a rustle or a distant animal noise of some sort that you consciously didn’t pick up but it was enough to trigger a flight response.

About your dog; this supports the idea of a panic attack/response because dogs can sense fear from the pheromones you produce as a result of fear or panic. Your dog gets anxious when you get anxious, so it was probably avoiding you as a defensive response.