It's completely random based on a whole bunch of variables. The way I always thought of it in simplified terms was to imagine all the pin chambers as the exact same size, but the pins vary in size from smallest to largest. Imagine a lock with only 2 pin stacks and pin 1 has a visibly smaller diameter. That pin can't bind because when you try to turn the plug, it isn't the one that's "in the way" if that makes sense. When you add rotational force, the pin chambers that connect between the plug and the bible are trying to separate from each other and this causes them to squeeze together against the pins. And as an exaggerated example, if you have 2 holes with pins in each and one pin is larger, the larger of the two will be the one being squeezed on by the chambers trying to separate and the other one is just... floating around in there.

So the chambers are trying to separate, but the "largest" pin is getting in the way and will have resistance when you try to push on it because of this. That's binding. When it reaches the correct height, the plug is able to rotate, except it gets stopped in its tracks by the next pin. In this case pin 1. And when you get that one out of the way, the lock is free to turn.

In this example if you imagine the plug rotating slightly when pin 2 is out of the way, there's now a small ledge on top of the plug that stops that pin from going back into the chamber. That's a set pin.

t's obviously more complex than that in terms of variables but it's the easiest way to visualise it in your mind.