Whio was one that first forsaken this relationship? I guess we all know the answer. (Players with active subscription had priority over inactive ones)
And merge was merge, not transfers. Everyone was moved. For some reason BW decided to give new servers names after ones that got closed. Probably to keep new servers more familiar, but it backfired with waves of people thinking they weren't transfered.
Actually merging is forced transfer. It's when you take one server and combine it to another. When you merge (for example) Corellian Run to Shadowlands then the players and data from Corellian Run are being merged (there it is) to the Shadowlands server (or database). Consolidating is what they did, where they combined these separate servers and made a new server to place these combined servers onto.
To answer what happened is just that, they consolidated several servers to create 'super' servers, so everyone had a fresh start. Like this poster I quoted pointed out, BW decided to name the new 'super' servers the same as certain old servers to create some feel of familiarity. All kinds of fail.