I've watched that occur for quite some time. It used to be that it was a downtick 90% of the time. While I don't have a concrete answer for it, it seems like a couple of people are playing games. One who wants the price to go down to show a negative slant, and somebody who decided to try to make things look a little better at the end of the day.
Seems like some sort of psyche game. I can't think of any other reason for the almost constant trades that occur that time of day.