Biology, Politics, and The Backwards Goblin Backstory

Why do the goblins in my campaign talk backwards?

It's simple evolution.

Our real-life perspective on neurobiology is predicated on the idea that humans evolved as the only intelligent life-form here on Earth. Our brains are basically wired to deal with avoiding, training, and preying upon dumber life forms.

However, what would evolution have cranked out if (like in most D&D worlds) we evolved next to other highly-intelligent, technology-and-language-using lifeforms and (like in most D&D worlds) the recorded history of civilized humanoids stretched back far longer than our real-world recorded history?

Well, we'd probably have all sorts of atavistic behavior patterns developed hundreds of thousands of years ago to help our primitive-but-still-civilized ancestors survive that are nothing but vestigial xenophobic weirdness now.

For example: Goblins--during, perhaps, their endless wars with the elves--developed a hormone which prevents them from ever telling the truth to another species. When Lighttouch Silverlegs interrogated Grungle Snumphungle at knifepoint and asked "Where doth thine catapults dwell?" Grungle had no choice but to say "Under a pile of carrots."

Of course, nowadays the goblins are more sophisticated and realize the value of both interspecies cooperation and occasionally telling the truth in order to more thoroughly deceive their enemies, but still, the verbal tic remains. Their biology--which served their species so well for so long--simply will not allow them to be honest with elves and people. Thus: they talk "backwards".
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As for why everything in the palace is on the ceiling, that's actually a whole other thing, and the answer is political rather than biological:

Despite having a king, the goblins are actually relatively democratic (teratocratic, I suppose, actually) for the middle ages, and refuse to let the king and his ministers leave the palace unless they actually do their job. So: all the palace ministers, guards, harem witches, etc. are bound to their own shadows--the shadows are, in turn, bound to the palace. An enchantment prevents the palace inhabitants from detaching from their shadows except on official business connected to running the goblin city's affairs.

So: they can only leave if they are carrying out a proclamation, legal decision, etc. Otherwise the government is essentially held hostage in the palace. Though it is, by goblin standards, a pretty nice place.

Putting everything on the ceiling makes it easy to spot intruders--anyone who doesn't belong in the palace isn't attached to their shadow and so can't walk on the ceiling.

Most of this information is, incidentally, findable in one place or another in the campaign. But like any good sandbox, there's no guarantee anybody'll find it.