I hope that this past answer of mine may be of use to you. The short answer is that William was likely low on supplies after his invasion was delayed by unfavorable winds, and almost immediately after landing he broke his army up to forage (read: loot) in Sussex. Harold then marched south very rapidly to confront William, which forced the Normans to hurriedly concentrate the army. Unable to either forage or remain in place indefinitely, William forced a battle at Hastings.