TL;DR ladder bugs are not random! They depend on map variable ground level and if they got 'fixed'.
Ladder bug historian here.
Ladder bug #1 is caused because the bottom part of a ladder, the one on the ground, and the rest of the ladder are actually two different 'objects'. The idea being that with sprite height you should automatically 'attach' to object 2. With maps being hand drawn this causes a situation where the bottom part ends before the rest of the ladder starts, throwing you off if you try to run on to it. Popular fix for this was jump on the second part of the ladder. After all these years I never got a developer to answer why that was the initial design probably because it's lost knowledge by now. Technically not a bug but bad design.
Ladder bug #2 occured when a underqualified yet motivated developer (I think the same who did the automatic dismount script, script I say not a different bad word) decided the best way to fix this was to raise the ground part higher. Thus every single ladder bug #1 that was reported to they/them was now a ladder bug #2. On those ladders, when you JUMP on the ladder you get thrown off. Not funny for those of us around for years conditioned to JUMP on the ladders to avoid ladder bug #1 but funny enough, each ladder bug #2 has a different height that jumping on will cause you to fall off, making it hard evidence of competence! It also happens organically in some post sharn packs with ladders leading out of piranha water or acid (in vecna optional for example) which in combination with jump lag + jump out of water lag + ladder bug #2 results in some very comical, video capturing worthy deaths. Technically not a bug either, just lack of competence.
DDO is awesome!