I had the same question about a month ago, as I'd decided to defer so that my boyfriend and I can start grad school at the same time.
There was nothing on the board, and virtually no info on the schools' websites, so I wrote a professional letter to each school I was accepted to, addressed to the medical selection committe, asking for the opportunity to defer for a year and why.
I thought about writing a letter saying that I wanted to defer so that I could travel, take classes, gain more work experience etc, but ended up just telling them the truth. Short and sweet- I'm excited to to their college of medicine, but want to wait one more year so that my boyfriend and I can start graduate school at the same time.
All of the schools were totally fine with my deferral request, but counseled me that once you have decided to defer to one school, you have to withdraw all of your other applications that are still out. So, the schools are just holding onto the letter until the end of the process, and the deferral is available if I need it.