How old is the dog? I'm far from an expert, but would think that the chasing is because the dog is seeing something unidentifiable (to him/her) moving. Distraction would be my suggestion if it worries you. When the dig begins the chase, call his/her attention to a ball, a treat or some other aactivity that will focuse attention elsewhere.
"Some days you're the dog; some days you're the hydrant". author unknown