It's... sort of both, actually?
Basically, the way the code is written, the intention is that each rock has a certain probability of uncovering a ladder when broken. However, the RNG in this game is often very predictable. If you know the location of a rock, the floor it's on, and the game seed (the number in the game's save file, in 1.5 you can set the seed to a specific number when you create the game), then you can predict whether that rock will give a ladder.
So although it's intended to be a percentage chance every time you break a rock, in practice you can create a grid showing which location will give a ladder, and then you just have to find one of those locations that happens to have a rock on it and break that rock. This would be very hard to do by hand, but if you're interested
@BlaDe has a predictor app (
found here) which will let you see ladder locations (among other predictable things).
Incidentally, the probability of getting a ladder from a rock increases as the number of rocks on the floor goes down (or you can think of it as new ladder spots appearing as you break more rocks) - if you have only two rocks left on the floor and there's no ladder yet, the next one that you break is guaranteed to give a ladder.
Killing all enemies on the floor also increases the chances of getting a ladder, and so does having a higher daily luck and increasing your luck buff.
It's also worth noting that killing enemies has a 15% chance of getting a ladder, which is usually higher than the chance from rocks. However, killing enemies often takes more time than breaking a bunch of rocks. So you have to think about how quickly you can kill the enemies (and also whether you want their drops).
In terms of mining strategy, keep in mind that if there's a random free ladder on the floor when you enter on the floor, no more ladders can spawn from rocks, although you can still get ladders from enemies. So you don't necessarily want to just go in and start breaking rocks willy-nilly, because you could be wasting energy and time when there's a freebie off screen. But you also don't necessarily want to walk around the whole floor to look before breaking rocks, because that also wastes time.
In single-player on PC, one (rather tedious) options is to pause and take a screenshot so you can look at the whole floor and check for ladders. If you aren't doing that, you have to find a balance to be found between walking around and breaking rocks.
(Incidentally, if you get one ladder from a rock, that also prevents other rocks from giving ladders. Getting ladders from monsters won't prevent you from getting other ladders, though.)
Note I said "random free ladder" in the last paragraph. Ladders that are always there (e.g. the ones on treasure floors in Skull Caverns, or at the end of floor 12, the long one with the minecart at the end) don't block other ladders from spawning. In fact, it's often a good idea to break rocks in Skull Caverns treasure floors even though there's a free ladder, because it doesn't take much time to break all of them and you might get a drop shaft.
Speaking of which, the above is all just how ladders spawn. The holes in Skull Caverns are pretty simple - every time a ladder spawns there, it has a 20% chance of turning into a hole instead.
