According to reports from the Athletic's Michael Russo, Hartman tossed his stick in the direction of the referee after he was irate because of a missed high sticking call. At this moment, it seems the NHL is debating between assessing a fine but leaning towards the automatic suspension.
The missed call against Hartman was honestly much, much worse than him tossing his stick afterwards in the general direction of the referee. Moments ago now, the NHL Department of Player Safety announced the hearing for Hartman will be held tomorrow.