Eeny, meeny, miny, moe, most likely. Alternatively they are forming a low pass filter with the parasitic capacitance, to round off sharp waveform edges so they don't have to worry about RF emissions - assuming about 15pF, that would be at 1/(2*pi*22*15pF) = 482 MHz - not really very useful unless they're doing USB 2.0, which I doubt. I'll go with eeny meeny.
They probably selected them with the same care with which they drew the schematic. (Sorry. Pet peeve.)