Thats not always a reason for walking though I dont doubt its common. Sometimes it could be where she hasn't misled you but hadn't shown a face pic or perhaps nerves or sudden loss of horniness.
Well if she hadn't shown a face pic and was not attractive that IS a reason to walk.
Nerves, well that's part of the fun, or if nerves kick in it's Prior to the girl opening the door, if the girl opens the door and shes attractive even if nervous you won't walk, its only if yoru nervous and disappointed at what you see when she opens the door, then it is a reason to walk.
Loss of Horniness, this doesn't happen till a guy has either shot his load or been faced with a Minger, both are again reasons to walk.