I'm a UK citizen with an ESTA. However, at MIA I got an X and had to queue up (much to the frsutration of my American wife!). The guy couldn't tell me why I wasn't allowed through, but does anyone here know if there's anything I can do to prevent this next time?

We live in the UK and I've never started the process of getting a green card.

EDIT: The itinerary on that trip was ex-DUB, not LON so maybe that was an issue?