It was mentioned before, but not forcefully as it should have been. The OP provided an incorrect procedure for "blocking" the wheel. He blocked (chocked) the wheel that he was lifting, which once the wheel is lifted off the ground and then removed, it provides no mechanical resistance to prevent the car from rolling.
When lifting a car from the jack point on the rocker panel (side sill) you ALWAYS chock the opposite wheel on the opposite side of the car (i.e. lifting the right rear means you chock the left front wheel).