The wilderness is all random encounters.
Very occasionally, in my part of the world, one will encounter a flock of waxwings. Only during the autumn or winter, usually only where there are rowan berries, and most often in the morning. They appear as if from magic at dawn one day before heading off to somewhere new, and indeed to ancient people it probably wouldn't have been far-fetched to have attributed their comings and goings to some supernatural force - like a sign of a coming storm, or an omen of death, or a symbol that the baby in one's belly would be a boy, and so on.
We now know that waxwings live in northern Europe and, when berries are in short supply over there, they pop over the North Sea to gorge on them here instead. How the waxwings know to do this, and what triggers it, are anybody's guess. The point is, as you go about your business here, particularly if you live in the countryside or have a big garden, now and again you'll see a flock of waxwings between October and March. It happens.
All wilderness encounters are, of course, like this. All of a sudden your path crosses that of an animal. Why? Well, because there are animals purposively going about their lives and occasionally they happen to be doing what they're doing near you, while you're doing what you're doing.
There is, in other words, nothing unusual about suddenly being confronted with the appearance of another living thing, apparently 'at random', when out of doors.
In the artificial and enclosed 'dungeon' environment, though, I've always thought that something stinks about random encounters. Unless the encounter is with a being that already exists within the dungeon key, and is assumed to be moving around (that is, if the encounter is with a being that is extraneous to what is already plotted), then one is forced to simply put out of one's mind the question of where it came from. Why is this giant slug, which the random encounter table just threw up, suddenly here? Where was it before? And why is it that it it does not appear to have had a material effect on its dungeon surroundings prior to this point? You will all be familiar with having to avoid this sort of uncomfortable question: how is it that this monster has suddenly come along, given that we know that the doors in the W and E exits of this room are locked, and the one in the N conceals a den of ogres, and we've just come from the S? Well, it was following you! So, can we follow its trail? And where does that lead? Er...
The only reasonable answer as to where randomly-encountered monsters in the dungeon come from is: from outside, or from further down. In other words, there is a strong argument for suggesting that the only principled and coherent way to approach the creation of random dungeon encounter tables is that the encounter table for each level should only comprise monsters that could have come from outside and whose trail will lead back outside somehow (whether through cracks to 'other caves' or to the surface), monsters from the dungeon key who are moving about, and monsters that could have come from the levels below. Either that, or go the whole hog on the 'mythic underworld' motif and assume an abyss or hell that permanently generates fresh monsters.