2000 Ford Taurus after repair wheel shakes

i just had my rear brakes done, new pads and rotors, as well as 4 new tires and an alignment. within one week the steering wheel started shaking very slightly when i put on the brakes. it seems to have gotten worse. i am wondering why this would be happening when i just had new rear rotors and pads put on, and it did not do this before i had the work done. i am taking it back to where it was fixed next week,and i'm concerned they may give me the run around and try to tell me other work needs to be done. and i wanted an opinion on why this would happen and is it resonable to expect them to fix it free of charge.
thanks