What are the problems with going out alone in the 3 weeks?

The Shulchan Aruch (Or. Ch. 551, 18) cites an ancient custom which warns not to go out alone during the middle of the day (depending on the locale, from about 9:15 am- 3:30 pm) during these 3 weeks (between 17th of Tammuz and 9th of Av), but this is only where there are no people around, like the desert or a forest, and unless one is going to do a mitzva. The source speaks of fear of a certain type of negative wind or spirit that's out there at that time while others associate it with the mourning period for the destroyed Temple in Jerusalem. Nevertheless, many modern authorities (like R. Shmuel Kamentzky) write that today, we need not worry about such things in the modern world, and Rav Kook adds, especially in the Land of Israel.