Ronon has to leave Atlantis for Earth for some reason (maybe to train the SGC soldiers to fight the Wraith, because that's apparently going to become an issue since Midway), but when he steps through the event horizon, there's a glitch.
Glitch causes temporal shift and shoves Ronon to the wrong end of the galaxy on the wrong end of hundreds of years later. He's on a backwater moon, on the Rim when he steps through an ancient, decrepit gate that flares to life for just a moment before powering back down again. He's stranded.
He makes his way to a rading outpost where Serenity happens to be docked. Mal and the crew are there to make some job, and the first person Ronon runs into is Mal, who notices he's a bit out of place and offers him a lift on Serenity. Hijinks ensue.
Bonus points for hooking Kaylee and Ronon up.