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.