Why does he quiver without me even touching him?

i've noticed that whenever i'm about to give head to my boyfriend that not only does his body kinda quiver.. but his also.. pulsates or wobbles suddenly. I haven't even touched him at that stage, but when i do his body jolts suddenly and he moans a little. Even when i do start to give him head, his still pulsates (not in keeping with his heartbeat, its kinda a random pulsation). I dont understand why this happens?