As I posted upthread, there is no requirement, in D&D, that a creature be invisible or camouflaged in order to surprise the PCs. It is possible simply to sneak up on them while they are distracted.No in-fiction reason for what before which rolling of the dice?
I thought the what here was "characters are surprised" and the "which" was the surprise roll. In witch case the in-fiction reason described is that there are someone trying to be stealthy? Even if ignoring the 5ed requirement we still need the in fiction reason that someone is in the vincity?
If the what is there are someone in the vincity, and the which is the wandering monster roll, then the in-fiction reason would presumably be that the characters have spent some time in an area that are commonly experiencing some traffic?
Consider, for instance, a situation where a group of 5 Orcs come around a dungeon corridor and surprises some PCs. Why did the PCs not notice the Orcs? If they had been listening, presumably they would have: the Orcs are (if typical instances) wearing armour and carrying kit. The obvious explanation is that the PCs were distracted, doing other things, or thinking about other things, perhaps talking among themselves, etc. The ability to avoid such distraction, and to focus intently on what is going on in the environs, is represented (in 5e D&D) by the passive WIS (Perception) score; in AD&D it is represented by the chance of not being surprised, which is better than the norm for rangers and monks.
But any such fact about the PCs being distracted and not paying attention to their surrounding is not established until the dice are rolled: in AD&D that is the surprise dice; in 5e D&D that is the DEX (Stealth) dice.
The point will even generalise to some cases of camouflage or similar. For instance, stealthy Orcs might try and match their movements, and the sounds they make, to other background noise with which they can blend in. But that sort of ambient background noise, which is something that makes Stealth possible, is also not established until the dice are rolled. If the DEX (Stealth) check succeeds, then the GM can narrate: "A group of 5 Orcs suddenly comes around the corner, catching you by surprise. You didn't hear them against the background noise of the dungeon." That background noise is established as a part of the fiction now to explain something that happened in the fiction back then.