Well, currently two main things affect it:
How much you drop at once, and from how far to zero this drop was.
Your combo multiplier, which is tied to back-to-back decreasing of the counter (or more accurately, getting closer to zero. So in the negatives, plus cards also affect the combo), and is broken by an increase of the counter.
Also we will remove the cap for the max allowed counter number in version 0.2