While pulling out of the hole with the first stand of 5” drill pipe to change the BHA, the 3/4” make up cable had been removed from the rotary tongs and replaced with a 3/4 “ snub line. During the change out, the make up cable was dropped down between the drawworks and snubbing post which was in line with the brake linkage. As it fell, the thimbled eye on the cable became wedged under the brake linkage. This was unknown to the driller. When the driller disengaged the clutch and tried to apply the brakes, the wedged thimbled eye on the cable kept him from doing so and the blocks fell to the rig floor. No injuries occurred as the floor had been cleared of all personnel.