!! Pure speculation, possible spoiler !!
If the Travler shares a connection to the Veil and the Veil is conciousness, Is it not impossible to beleive that when the Travler created ghosts, that it created a vessel for the conciousness of those connected to the Veil?
This is probably how the Witness was able to create a link through our ghosts.. by using the Veil, it was able to use our ghosts to manipulate the travler into creating a link.
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The Witness got its fingers into our Ghost way back in Shadowkeep when we boarded the Lunar Pyramid. This connection has been maintained when in proximity to the Pyramids. This is not every directly explained, much like how the narrative of Garden of Salvation relies on you paying attention to the colour of motes to figure out what’s going on.