Waiiiit a second ...
"You´ve got caught with a flat, well, how ´bout that?"
How does Frank know Brad and Janet have a flat tire? Brad just says
"Could we use your phone? We´re both in a bit of a hurry. We´ll just say where we are and then go back to the car."
He didn´t say what happened. (Notice the lift of the eyebrow to the audience, when Frank says "How bout that?", as if he had a plan for them and makes fun).
And: Riff Raff sings in the end "Your mission is a failure" to Frank. We never know what that mission was, but maybe Frank, Riff and Magenta and maybe even Columbia, who are there in the beginning during the wedding scene (and marked by the criminologist on the photo!) had a plan to lead Brad and Janet to their castle, for whatever reason.