These are the lot where the whole fleet disappeared and the new codex goes into greater detail of what happened.
To recap, the Imperium went all beslubber you guys at the end of the 3rd Phase Expansion and set fire to the Damocles Gulf. The Tau, not having much in the way of advanced drives available found themselves without a direction to expand what with Tyranid ravaged worlds, Ork invested hell holes, and the galaxy's greatest barbeque being the most viable options.
So in true Tau fashion they decided to screw around with new drive types. Enter the Slipstream module (someones watched Andromeda) based on captured Imperial and Kroot technology. With this the Tau were able to go further and faster. Tests worked fine so the powers that be said, beslubber it, build an entire expansion fleet with these drives and lets go all propaganda with the launch. Scientists wanted to whoa here, slow down, we have no idea what will happen with hundreds of those drives all firing off at once in one volume.
We all know what's about to happen.
So they launch the fleet and all those drives going off at once made the whole fleet go Event Horizon and vanish into the TEAR IN THE WARP they had just generated. Whoops. All of this broadcast to every Tau world.
The fleet's gone.
Fast forward X number of years. An antiquated drone turns up in the area where the Tyranids had gone all nom nom nom. It's from the disappeared fleet and back tracking it leads to a worm hole as we've now gone from Andromeda to Event Horizon through to Star Trek Deep Space 9. On the other end of the worm hole is the lost fleet where they have *not* had a good time of it. They were trapped in the warp, as one does, until *something* intelligent ripped them out the hard way thus creating the worm hole. These Tau are not completely right in the head due to the experience. They *hate* aliens, including their supposed allies. They're mean. Their own allies are all gone and they won't mention what happened to them but we're all pretty sure it happened at the end of a pulse rifle. Three new Septs are subsequently created by the old and new Tau.
So, the fleet was saved by an intelligence in the warp. Begin your massive wild guessing as to whether it's Tzeentch, the Emperor, some Eldar delinquent and so forth.
Spoiler for a Farsight book - the opening shows Earth and Water castes beslubbering around with an Imperial drive unit. It all goes wrong, as it does, and of course the Water caste guy is quite obviously possessed by a daemon. A Tzeentch daemon who can only tell the truth. The books a hoot as there's also an Ordo Xenos Inquisitor undercover with the Farsight fleet.