@gollum,
Firstly, I'm sure you've heard of some doctors who might not be so ethical who are pill-pushers. This is fortunately a small percentage of the medical population but certainly exist. Sadly, some docs will supply some patients with whatever they ask.
Secondly, prescriptions get stolen all the time and/or illegally obtained scripts are not uncommon to segments of the public who are determined to get them.
Thirdly some people, elderly and infirm, haven't the concentration to be certain they're observing proper dosage. As someone deteriorates due to various infirmities, they simply take an overdose.
Fourth, mistaken medical prescription or interaction issues with other meds - leading to either on overdose or toxicity
.
Lastly, doctors make mistakes and change medication dosage but sometimes fail to educate their patients with infirmities. Often times, the person who might be helping the infirm patient might not understand there was a medication change. Poof..another med overdose.