The wake vortex or vortices can give the envelope quite a shaking. The crown of the balloon is sealed with a parachute valve that can become dislodge and allow the hottest air to vent leading to an unscheduled descent. Re-seating the parachute is no big deal and a bit of burning restores the lift however it's poor airmanship all round.
If you know any balloonists then trading an hour for an hour is always very welcome. Navigating at 10kts using OS maps is a rather different experience and skimming the top of the barley can be terrific fun too
