I've done some searching to make sure this hasn't been mentioned before. Not that I've seen. Basically, if you kill multiple monsters at once, and one (or the first?) of those kills would have caused a level up, each additional kill of that round can also trigger an essentially "free" level up. I'm not sure how long this has been around, as I only really started dabbling in Promethean after the New Year. An excellent place to see this is at Monster Zoos when you're hurling fireballs in there. I've gotten anywhere from 1 to 4 extra level ups from such a situation on several occasions. Note that different spells do seem to track kills/level ups/experience differently. For example, multiple kills by fireballs or explosive runes can score an extra level up, but multiple kills by dragon's breath (or at least the 3x1 trail it makes) do not seem to. Presumably everybody's damage is calculated simultaneously for fireballs, but dragon's breath handles each target of the fire trail one at a time? Pure supposition on my part. Note that I haven't tested other skills that can damage multiple targets at once, so it could be related to certain fire effects only. While it's certainly...interesting...getting a bunch of level ups at once (and it can take the edge off a recent deep-dungeon permadeath), it also spoils the fun a bit. I've actually fallen back to autosaves just to try to avoid the benefit of such actions (only to have it happen on some other occasion because, dangit! fireballs are handy).
Actually, this is a known bug, but there's no harm in bringing it up again. Your theory that some skills are capable of triggering it and others aren't is an interesting one, and deserves further testing.
Having picked through the relevant .xml myself while working on a skill mod, I'm pretty sure that this is entirely correct. Spells like Obvious Fireball are, simplistically speaking, "Deal this damage to every tile in this predefined area (and then with the burning aftereffect)." The damage is dealt to each tile, all at once, since it's all part of the same effect of the same spell. For Dragon's Breath, the area in question is really only the tile immediately adjacent to you -- and then it calls two more spells, in sequence, which hit the next two tiles in line for progressively less damage. The spell for the third tile is also what calls the "set the dungeon floor on fire in a 3x3 area" spell. This is also why, if you're paying attention, you might notice that the flame animation for Dragon's Breath plays through on the first tile before starting on the second, and finishes on the second before starting on the third. (If you're not paying attention, the game tricks your brain into smoothing things out by cutting out a few frames from the start of the first two tiles' animations ... and using a high enough framerate that the whole deal is over in a half-second regardless.) Since each tile is a separate spell, the game checks for kills/xp gain in between each effect. You also won't see a multi-level-up if the kills come from the aftereffects of the area-of-effect traps, since that damage isn't calculated until the start of each respective monster's turn.
