There are many reasons that a horse does not like to be clipped.
Some horses are frightened of the feel of the clippers, others of the noise.
Some horses have had bad experiences in the past and resist because they associate those unpleasant experiences with the clippers. It's hard to blame the horse for acting up during clipping if the clipping is hurting him. Therefore you should make sure both your clippers and technique are in good working order. In order to minimize potential problems, make sure the clipper blades are sharp. The horse should be clean and dry and the clippers should easily glide over the area being clipped.