Back in the late 90s, EA rereleased Ultima IV as freeware. Sort of. If you didn't download it from an officially authorised host, you're still technically committing piracy - even though the game is indeed freely available. It's a long story, documented exhaustively here, but the sudden upshot of it is that EA are cracking down on sites that have been hosting it for bloomin' ages. Unfortunately, this has also affected a number of ambitious and long-standing fan mods and remakes.
Why? And why now?
Well, the theory goes that it could be related to whatever secret project it is that Mythic are currently working on. Hints have been dropped that it could be Ultima-related, and a great many fans seem pretty convinced that's the case, but there's nothing even slightly like confirmation. You can browse all the rumours and speculation to date here.
Whether this has anything at all to do with the spate of Ultima IV removals is anyone's guess - it could simply be standard IP protection stuff, after the right/wrong piece of paper happened to float to the top of a pile somewhere all of sudden.
The good news is that a few places (such as this one) do still host the original Ultima IV, at least for now. Get it while you definitely can, I would.