Sometimes, someone who has had a drug addiction or problem, manages to break free of it and live a clean life, sometimes for months, sometimes years. Then they run into someone they used to know or are offered something they used to take and I think they believe it's been so long it won't hurt them now if they just have one "taste". Sometimes they will take the same amount they used to use and it is too much, their bodies aren't acclimated anymore.
This is all just my own humble opinion. I have a friend who lost his son (who I also worked with) and that is what we assume happened. Because you try to rationalize and understand how it could happen. He had been clean for about 2 years, working, engaged, going to the gym regularly, eating right, taking care of himself - and it was a huge shock when he was found dead from an overdose. Only in his mid 20's. It was about 6 years ago now - but it pretty much destroyed the family. My heart just broke for him and for the family he left behind.