There was an Athletic story about this that was pretty interesting. He actually asked for a number change over the offseason but he wanted 14, which Adam Duvall took. 45 was assigned to him and no one from Atlanta was willing to talk about it, so it seems likely someone in the organization decided to be a jokester and bring that topic into baseball for no good reason.
Tromp himself said he didn't even realize for a while why fans were giving him more attention than usual until it was brought up to him.