I'm not familiar with the DJX. Did you try to download a manual for it?

Some Yamaha keyboards had a mode for "Smart chords" which allowed you to trigger the most common chords for a particular scale or mode by pressing only one key. Since in the key of C major the sixth degree is minor, the A-key gives you A-minor. Likewise the second degree is D which is also minor, hence D-minor. Very convenient and easy to play as long as the song stays within a single scale / mode.

This was a feature on the PSR-225 and PSR-270, probably some other more recent boards. Not sure about the DJX. Your best bet would be to refer to the manual.