From everyone I’ve come across addicted to something like that it’s always been life circumstances spiraling out of control that pushed them to it. The study itself acknowledges that “Nearly one quarter (24%) noted that substance use currently them health, legal, or financial problems.”
The 31% number specifically is in the following context: “Many participants reported using drugs and alcohol to help them cope with the circumstances of homelessness.”
It’s likely a mix, but life circumstances are almost certainly going to be the cause for most. Living on the streets is hellish, of course people are going to look for some temporary escape
Flawless logic