They merge servers weirdly.
First they pick the servers that will receive a merge not on the server's activity, but rather by what servers ask for it. On top of that they take the word of paying players much more than non-playing. So if your server has a VIP in it, and that VIP feels that your server needs a merge chances are that you are getting a merge.
Then there is how they choose who gets merge with who. I have no idea how they do it, to me it seems almost random. My server got merged with a server that was much much stronger th*, and another that was much weaker th*.