It might be one of the causes, but it's sort of naive to consider it as the main one.

SF is not the only big city with a major influx of people coming there for work, I have seen quite a few European and American major cities, but "armies" of homeless out in the open like in SF I really haven't seen anywhere else.

It's probably a mix of causes, not least the very mild climate, if I were homeless I would rather spend a winter on the streets of SF rather than in Boston or NYC, or a summer in Houston...