!! 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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Drifting off the point: How many guardianless ghosts are still fluttering about do we think?? Since our rebirth there's been one guardian and a -blam!- ton of hive (who all managed to pop to eververse to get their ghost shell* updated). *it bugs me that savys ghost is pre-hive themed on first res... was there any lore to say how it got all hived up?? Did savy round up all that could be found, and they were used when the time was right... enacting a Stockholm syndrome type thing during that time?