Acepincter is correct. It’s sort of a centrifugal force in which the denser air resting at the bottom of the elevator gets moved upwards due to the change in downward velocity.

You can do a similar experiment with a balloon in a car. Go fast, hit the brakes, the balloon moves towards the back of the car. You can find an explanation here.

http://www.teachersdomain.org/resource/phy03.sci.phys.matter.buocar/

Their reasoning is opposite my own (they sue acceleration instead of deceleration) but it’s still sound.