When you enter Wonderland there are two kinds of Descent: controlled and uncontrolled: uncontrolled descent is like falling down the rabbit hole, and survival is a matter of luck and chance; controlled descent is like crawling through a mirror, and survival is a matter of skill.The main idea that I'd take from this setting isn't so much the Wonderland set of existents (rabbits, mad hatters, cards, etc.), though they aren't alien to a D&D setting; it's the conceit of Descent to the 7th level of reality.
When a person descends from level 0 to level 1, two things happen: 1) a simulacrum of that person takes his place in our reality and tries to emulate whatever its primary is doing; 2) the primary person is transported to a world that is similar to ours, but weird and twisted in small ways.
If that person descends yet another level of reality, the world gets even more twisted and less similar to the known world (metaphorically illustrated in the Eric Lofgren image to the right).
As you can imagine, a normal person dealing with a world that is weird and twisted in subtle ways would do things out of the ordinary -- some might say crazy. Unfortunately, this prompts the simulacrum to act out these crazy bits in the normal, sane world, often doing wonders for that person's reputation and standing with the law.
Now, I'm not saying that every adventurer that descends a level in a dungeon is actually entering another level of reality, and his/her simulacrum is doing crazy things in the otherwise boring cave above -- that wouldn't be very heroic, now would it? No, what I'm saying is -- this may be what drives the various denizens of dungeons and the Underdark to build traps and puzzles and chess sets and magical archways and mirrors: a reaction to the slow infection/infestation/invasion of the entities from this other reality.
As for the mad wizards that create and maintain mega-dungeons, I AM saying they are simulacra of these powerful wizards. The real wizards (the good ones and the evil ones) are in some other level of reality -- perhaps locked in an endless attempt to return to our reality or valiantly trying to keep whatever is trying to invade our reality on their side of the mirror. And maybe, just maybe, those moments of lucidity that Halaster Blackcloak has are the brief moments he manages to return to level 0 before being dragged back down.
There's a lot more you can mine from these two books; I highly recommend visiting the JAGS website and taking a look. You'll look at Gary Gygax's Dungeonland modules and all those Dragon magazine chess-inspired covers in a different light.