Several therious are about on how this started. The early egyptians would place a ladder in the tomb for the deceased to climb out and into the after life. When they placed it next to a wall, it would create a triangle, and also was supposed to represent their version of the Holy Trinity. So walking under the ladder violated the sacred space.

Then in the midevil times, being under a ladder often ment you were hit with boil oil or rocks.

Then later in time, a person was walked to the gallows under a ladder, before climbing up the ladder to be hanged.

Where do you think this superstition has come from?

Aren't there some American Indian tribes who also placed a ladder in or near a grave for similar purposes? I seem to vaguely remember this being the case.