The horde that had gathered had not always been a horde. It started as a smattering of zombies across the countryside. Most of the zombies had risen out of graveyards throughout England from the initial uprising and still sported their tattered and filthy funereal best.
The excitement and motion of the Pemberley event acted as a beacon to zombies in the area.
The problem was exacerbated as guests started to arrive from a variety of places by carriage, which set zombies into motion towards the estate.
Zombies, however, move extremely slowly and only just arrived after the gathering was well underway.