I'm going to go out on a limb here and blame weed. Because someone has to.
Seriously, look at all those free living bohemian types, where there is a big crossover with the anti-establishment "weed isn't a dangerous drug". And then 10-20 years later there they are, crazy as fuck. Smith is joining the ranks of Lucas, the Wachowski Brothers and many other artists who have completely lost it.
I get it, weed has been oversold in such works as "Reefer Madness" but that's become a straw-man, I don't think weed is good for the long term mental health of artists. And it's subtle, the effect.
I am very much against medicating for ANYTHING, I've seen too many people who start trying to use chemicals to guide their lives and ended up taking them where they didn't want to go.