Lore wise the trolls used to be damned near everywhere, Arathi Basin and the Hinterlands both had there fair share so it's not a stretch to think that Southshore did as well. I'd guess it's probably exactly what it says on the tin, the very early stages of a future troll raid, the whole expansion is time travel, so why not pull a CoT raid out of thin air? It certainly seems more plausible than a 100vs100 BG, which when considering queue times, performance, and certainly not least compelling enjoyable gameplay starts to look pretty silly.
"get at least level 40 before you come here"
The nostalgia.
But personally I really dont care where we fight as long as we get a massive battle once again.
Tho TM vs SS sounds awesome of course