Another reason for a large amount of overdoses is drug mixing - people taking oxy and then xanax and then they're dead. But I think your comment and the guy above you both refute hapless's claim that most people who die from an OD are simply misjudging their own doses. In truth, I believe very few people misjudge their own doses. Drug addicts are notoriously good at knowing the exact amount to get them high. Why? Because too much is a waste (they try to extend their supply as long as possible) and too little won't have the desired effect.

In addition to your reason for ODing (use after a period of abstinence), the recent rash of ODs in my state are because dealers & suppliers are cutting their dope with fetanyl and other stuff that the drug user isn't/can't account for.