I appears to be based entirely on the number of recent (within the last 2 days) replies. View count does not seem to factor into the algorithm. It goes "cold" when there haven't been any replies for a day or more.
This is based purely on observation, not inside knowledge. You could, however, empirically validate (or debunk) my hypothesis by reviving any ancient thread about nothing and then appending a bunch of becan jokes to it.