The rolloff is probably the cut-off filter of the RMAA at 20kHz, it is a very sharp slope. See the uDAC measurement as part of the 25 USB DAC plots.
I've seen plenty of 16/44 RMAA measurements and all of them extend to 20khz without such a roll-off unless there is something flawed with the output or input device (e.g. on an older computer with a onboard sound chip). If there is a filter it's beyond that point. What is going on with this measurement?