The only reason to issue this section as a prologue, rather than the first part of chapter one, is because it works better in the present tense, but I can't pinpoint why.

It's setting the scene in a way that is quite seperate to the tone of the rest of the story, and although I've tried twice now to move it to past tense it just seems to lose something important and I wind up deleting the past tense version.

I wish I understood it better and could explain it. It certainly isn't following any kind of logical rule about when it is, compared to the later action, and I hate that.