Thomas and Martha met during her journey to save the world in the season 3 finale of Doctor Who, Last of the Time Lords. After time was reversed at the end of the episode, Martha found Tom; he was a pediatrician working in Africa. They began dating off-screen, and in the fourth episode of season four, The Sontaran Stratagem, it is revealed they are engaged to be married (which Martha seems to be very excited about).
DONNA: She's engaged, you prawn.
DOCTOR: Really? Who to?
MARTHA: Tom. That Tom Milligan. He's in pediatrics. Working out in Africa right now. And yes, I know, I've got a doctor who disappears off to distant places. Tell me about it.
Then, out of nowhere, it is the last David Tennant/R.T. Davies episode The End of Time special and Martha is married to Mickey!
Did R.T. Davies or Steven Moffat ever address or explain what happened?