A leaf spring suspension absolutely NEEDS some way to positively stop the pinion from rising. Could be traction bars, ladders or just a snubber. The shocks mounted ahead of the axle are a help, but they are not forward enough to have an appreciable effect...not enough leverage.
The pinion is rising against the front of the spring...it reaches a point and then the spring does what a spring does..slams it back down. Once the spring is unloaded, the pinion begins rising again. Cycle this a few times a second, and there's your axle hop. The front end rose until the spring overcame the rotational torque of the axle and then just unloaded.