The last census I saw of the Los Angeles homeless population found that the percentage of homeless people in LA that were never housed in LA was higher than either the percentage of homeless people who had a severe mental illness or substance abuse issue.
My understanding is a lot of it is voluntary migration. If you you're not paying rent, you don't have a job, and you've burned all your social support ties, you might as well get a greyhound to a place with good weather, good social services, and where people will look the other way on your visible symptoms of mental illness and drug addictions.
