It also may be that miners are shifting from one DGB algorithm to another
The only GPU mineable DGB algos are Skein and Groetsl - SHA256 has tons of ASIC options, Scrypt has quite a few, and Qubit is mined by the Baikal ASIC.
I doubt this is a significant factor - more likely folks shifting from DGB to other coins entirely as the profitability bounces around.