I think it is a combination of the the sheer volume of SxSs' along with the lack of experience by many SxS drivers. A good way to gauge this is by who is or is not using basic hand signals out on the trails. More and more riders do not use them and a lot actually just wave at me when I am holding up fingers trying to let them know how many more riders behind me. That's a little scary.
I used to love to ride my dirt bikes at places like Gorman, Dove Springs, Spangler, etc., but the number of SxS's has grown at least tenfold over the past 4 or 5 years. It's just plain dangerous anymore out there. Now I pretty much stick to Rowher Flats for dirt bike riding since the trails at that place are not really SxS (or even quad) friendly, at least for the time being.