What are some reasons as to why the Tau would be floating about in Rogue Trader's default setting? As to my knowledge, correct me if I'm wrong, the Expanse and the Tau Empire are a good stretch away from one another. Is there some piece of alien-tech gubbinz such as the Jericho Jump Gate that would place them in the expanse? Is there a Hunter-Cadre floating about?

Do you happen to own the DLC "Twilight Crusade," "Shedding Light," or the "Tau Character Guide?"

Only the Tau Character guide!

The Soul Reaver adventure also makes reference to some Tau that were captured by the Dark Eldar. When my characters did that adventure I expanded it so they had captured a Tau exploration vessel and brought the survivors to the Nexus, where they were eventually freed.

As for a mcguffin, nothing official I know of; haven't bought the Tau pdf for RT. As for how they might get there, if the Jericho/Maw Gate exists, there can certainly be more of them, and then there are the various gates that go with the Warp Gate Map ship component, in Hostile Acquisitions (p.75). If you need them there, it is certainly doable, and as for why, simply to expand, if nothing else, and the Tau would find precious little to obstruct them, in the Expanse, compared to various other places they might crop up, as the Imperium, while "close by", is notoriously weak in the Expanse (which allows your players to be the devious little ________ they want to be, if they want to be).

I needed them for the story I'm writing, and originally, I planned to have them find another warp gate, akin to the J/M device, which would lead to the space just prior to the Rifts of Hecaton, but then I decided to allow the Eldar to meddle, and they used the Webway to go find some Tau, out in the Reach, and bring them back to the Expanse, in a region the Imperium ships don't like; the warp-racked region, near the Rifts of Hecaton. exactly what the Eldar want of the Tau, besides available troops, to cover their dwindling numbers, is uncertain, but so long as the Eldar aide them, they can move about via the Webway, and the Tau can't yet double-cross them, if they intend to, as they need the Webway to function. As for the people in the Expanse, the rest of the Eldar don't like what this splinter faction is doing, so they get my RT to fight it, he gets the Koronus Battlefleet involved, and one of Ebongrave's Inquisitors even comes to "assist". So yeah, bringing the Tau in can be very easy, if a little destabilizing.

It really depends on what you want from the Tau in the Expanse. If you just want a brief interaction with them and your players, then maybe several thousand of them have been brought in by Kroot War Spheres and other RT's coming through the gate. Maybe they set up a small little outpost just gathering Intel. On the other hand, if you want them to be a dominate force, then maybe they have a backdoor like in Venkelos's concept. Or maybe they are kicking so much ars in the Jericho Reach they have already assaulted through the gate and a large force broke past the Imperial defenses and are running amok.

P.s. I don't own any of those books, so my ideas are just from the stories I've heard or implemented myself.