Miners in North East England worked the coal face wearing only old breeches, boots and fibre helmat, the work was hot and sweaty and there was usually much water so pants were cut off just below the knee so as not to cause extra weight when wet. These breeches were called "duds". the flat cap was called "dut". So if the blackleg miner had his tools and breeches stolen and thrown down the pit it was a sort of symbolism that it should be him who was thrown down.
Cheers Bill