I too think that one of the reasons of a decreased score is the summer. I for example am utilizing (since ~May) only ~6 cores out of ~50 I have. But temps are going down recently, so I hope to resume soon.

The other factor I believe was HCC1 on GPUs in the past, where people earned lots of points and this has brought more folks to the team. Remember our scores and position then? With the end of HCC1, lots of folks were hoping for another such run, but they lost illusions after months passing and still no other WCG GPU project in sight. After experiencing the power of GPU computing and WCG still relying on CPUs, I believe this has eventually caused a shift to other projects.
I too said said to myself a few times why to bother with CPU computing if there's so much more that can be done with a single GPU... And still waiting for a new WCG GPU project...