I've noticed that all (or most) of the forts in the SI start with the letter "X". Is there a reason why or is it just because?

I wish I could give an educated answer. I've never thought of it before. I guess it may come with knowing about the Plane of Madness, Sheogorath, and the Daedric Alphabet. I wouldn't know, though. X and Y are the least used letters, though, so it's surprising. (As per Daedric language)

I don't think there is any lore reason for it, just a part of the look and feel of the Shivering Isles. Excessive use of the letter X doesn't indicate madness but is commonly to create a feeling of alienation.

You can compare it to the names of the Dwemer ruins. IIRC they used Sumerian names with quite a few letters replaced by CH's and z's.

Never had the pleasure to play SI thus far, but in some languages "X" is pronounced as "Sh". Maybe it's an obscure reference to the lord of the Realm?

Actually, I'd say the fact that X is used less often than other letters, especially at the start of words, is why it SHOULDN'T be surprising, since the Shivering Isles is a place that is different from other places and quite alien, it's only natural that this would reflect in the names, I think.

I'd imagine that's really what it is, I doubt there's any real lore reason behind it, it's probably just a matter of feel, whereas Bethesda designed Cyrodiil to be generic and familiar, the Shivering Isles was meant to seem alien, so I suppose Bethesda decided to use lots of names that go with the feel they sought to create. If there is any explanation, I'd guess that Bethesda first thought about the feel they wanted to create and then tried to find a way to explain it, as opposed to the opposite of this.