Blizzard have clarified a few things about the mount and bag drop chances, from the event as they have higher rates for the first kill of the day. 

Crossover Event (Source)

Just a quick callout on two of the items available during this event. The Tyrael’s Charger mount and the Horadric Haversack 36-slot bag each have a significantly higher chance of dropping on the first Treasure Goblin kill of the day on any character on an account. Subsequent kills or kills on alts have reduced chances for these items. All other items have the same drop rate for each kill.