I truly have no idea what the situation actually is, but here is an, IMO, plausible explanation: policing practices around homelessness have changed as a result of changing attitudes toward the homeless (toward increasing acceptance) and this has created an environment in which the homeless are now able to accumulate possessions and erect semi-permanent shelters. Another, plausible scenario (again, IMO): if certain liberal cities have a reputation for being relatively accepting toward the homeless, there may be a simultaneous net migration of homeless from less accepting parts of the country to these cities.

I'm confused... Are you saying homelessness can't be a mental health issue because they now have tents? Or because they have possessions?

Not at all. I even stated directly that in the recent past most homeless people had mental health issues. Now added to this first group are people that just can't afford the rent and have been evicted or pushed onto the street.