It's possible and even likely the lowered hash rate is caused by cpu/gpu miners giving up.
15,000 GH/s is
ALOT of GPU miners and and even bigger number of CPU miners. For them ALL to coincidentally go offline within a few days of each other is unlikely, in my opinion. Using Occam's razor, I am forced to lean towards the fluctuation being caused by a major producer of Hashing equipment. But who really knows... that's the wonderment of the
BTC ecosystem
-n0c