While inflating the head with the air pressure while doing Jalandhara I experience hopping at the buttocks, which is quite distracting. I couldn't find this described anywhere, the only hopping described is during Samyama. Is this normal, and is there a way to avoid it?
You can either stop the practice for a week and then resume it or reduce the number of repetitions in a sitting and observe the difference. If not, you got to sit in a softer surface to minimize damage.