They don't need to collude explicitly. They just watch posted prices in their region and match that. Since the posted prices are always above average (you start higher than the old rent and keep lowering it until somebody takes it), rent keeps going up for everyone.
Rents don't always go up even nominally; you've just listed the upward pressures without the downwards ones. I believe they're still down in SF compared to last year.
Posted prices can be misleading because they prefer to give discounts (X months free) rather than lower the sticker price, so it also depends how you count.
