The Club had learned that two distinct "dynasties" of the Thargoids, the Oresrians and Klaxians, had become involved in a civil war. At some point, the Klaxians had gained the upper hand, forcing the Oresrians into a slow, continuous retreat across the galaxy, a retreat which would take them directly through human space. The Oresrians intended to use human civilization as a "bullet shield" to occupy the Klaxians' attention while they escaped further into the galaxy, a scenario which posed a potentially apocalyptic threat to the whole of humanity.
http://elite-dangerous.wikia.com/wiki/The_Club?comments_page=1
I reckon the barnacles appeared as part of a supply route/vanguard before the full retreat of the first thargoid faction. They strip mine the planet for materials which are then harvested by thargoid ships (which can be observed in game)
They're the ones planning to run through oir space.