Why this individual broke away from the herd and where is it today is a mystery. Maybe spooked by a dog or human, or chased away by a competing male... What isn't a mystery is that deer populations are growing and they are reported more frequently throughout the 5 boroughs.
Read more about this trend in this NY Times blog article from last October:
http://cityroom.blogs.nytimes.com/2011/10/05/deer-are-found-on-brooklyn-coast-in-disturbing-circumstances/