What if Shemis is where the Gallifreyan Muses died? What if some essence of them had managed to survive their deaths? What if someone tried to create living beings using those essences - in other words, the trio?
SG, then, would be here to commemerate their deaths with the spirit of Shemis.
The Bosses would be after the trio because of some side-effect of the original bio-engineering - not their musely qualities, though, because we've established the Bosses aren't interested in those.
How's that sound?