Am I to understand that you have a new tenant in the unit? I've seen washer stand pipes that have run clear before, suddenly back-up and overflow because of a change of (or too much of) laundry detergent and the line gets suds-locked. Often reducing the amount of detergent or changing brands will reduce the suds and the line will go back to draining normally.
Try running a full load, no soap, no clothing, just maximum water in the maching, run the spin cycle and see how it reacts. If it drains, it was suds problem. If it still backs up, call a local rooter guy and have the line jetted. Snaking will clear a clog but only jetting will remove the scale on the inside of the pipe.