you're not characterizing overdose correctly. it's often people coming from pharmaceuticals and not knowing what they're doing, or not knowing the potency or contents of their drugs.
My ex-girlfriend died in September of this year from a heroin overdose. She was in the process of getting clean and then relapsed.
http://www.businessinsider.com.au/philip-seymour-hoffman-ove...
https://www.thefix.com/content/high-risk-relapse-how-going-b...
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.
One of the more common "mixes" that also happens to be particularly dangerous is drinking EtOH while taking an opiate (or other depressants). Unfortunately, as beer or liquor holds a different cultural status, it isn't always listed with other drugs.
If there is any opiate involved, it will invariably be written up as a "oxy/heroin overdose" even if the main problem was the 10oz of liquor consumed the same evening.