The triangle, that a ladder on a wall makes, has been symbolic of life since ancient Egyptian times and disrupting a very, very important symbol was seen by the earliest civilizations as tempting fate. Even in our secular age it seems like an unnecessary risk to walk under a ladder from which a pot of paint or scaffold who’s lost his balance might easily fall.
There is an alternative source for this superstition however – the medieval gallows. Until the late 1800s the ‘short drop’ method was used for hangings, which meant that prisoners were hanged from a cart or simply made to step off a ladder with the hangman’s noose around their neck, which usually resulted in death by strangulation. Later, when new drop gallows were introduced, which caused a quicker death by breaking the prisoner’s neck, ladders were propped against them so that prisoners could climb the scaffold ready for the drop. These were used again by the executioner when the bodies were collected. It was widely believed that the souls of those who’d been executed loitered under the ladder (since their crimes made them unfit for heaven) so it was inviting misfortune of the most grisly kind to walk underneath one and mingle with them.