This is copied directly from the DRM User Guide:

"For ionospheric propagation, the most severe Doppler spread is
observed in the case of Near Vertical Incidence Sky-wave (NVIS)
propagation. Because the path lengths between transmitter and
receiver are quite short for NVIS, the distances between the
ionospheric layers represent a larger proportion of the total path
length. The several reflections can also have similar energy levels.
This gives rise to significant values of Doppler spread.

"To counter the effects of Doppler shift and Doppler spread the
frequency separation between the OFDM carriers in the DRM signal is
progressively increased (e.g. the carrier spacing in Mode D is more
than 2.5 times that in Mode A). This ensures that the frequency
spread experienced is kept to a sufficiently small fraction of the
carrier spacing to allow correct demodulation."