There are many. It's an umbrella term for a range of circumstances that tend to be correlated with poverty and social issues.

Low trust in society, few opportunities to improve economic situation, higher prevalence of trauma and ptsd, higher probability of substance abuse, low opportunity cost for going to jail, fewer good role models, worse self esteem, worse education outcomes, worse physical health, higher likelihood of being involved in organized crime, higher likelihood of depending on parallel social structures for safety and protection, etc.

Each can be cause or effect in a self reinforcing network. Picking one single root cause isn't really possible.