Horses are flight animals which means that they are wired up by nature to run away at the first sign of trouble. Wether that trouble is real or imagined. They are also deceptively big animals and can be very difficult to control once they have got it nto their heads that it is time to get away from some source of danger.
One of the sorces of percieved danger that the horses in this area may come accross are the hot air balloons that regularly fly in good weather from the Black Horse Pub in Great Missenden.
Balloons are also big and heavy but despite being heavy they float in the air and go wherever the wind blows them. There is curently no no way to control the direction of flight of a hot air balloon (although a mighty ingenious farmer / balloonist not far from here is currently working on plans to invent a directional control device for balloons).
The only control the pilot has is wether the air in the balloon is hot enough to rise or cool enough to decend. Some of the large balloons can take quite a while to warm up enough tp change from level flying or decending to climbing.