Probably because I would suspect the number of people playing is starting to die back (gotten bored, moved on to other things.)
I also think the ending DLC was hanging over a lot our of heads as a "thing that was coming" and kept a few of us - like me - playing a bit longer while we marked time as it were. Certainly, I've noticed that since the ending DLC, I've been less inclined to play...
Also, of course, it's summer, and people are likely to have less free time at weekends to play because of holidays and such.
That'd be my guess, anyway.