The Sheppard/Beckett thing is IMHO a statistical anomaly, because there's one (or two?) people who are writing right now. It's one of those waves. On Wraithbait (which cover a much longer range of time) McKay/Beckett is the second most popular slash pairing, and almost as popular as Weir/Sheppard. Which actually makes me think that the Weir/Sheppard people are archiving somewhere else, or not at all, because there should be more W/S stories than that: currently Wraithbait has only 166 W/S, compared to 1350 McShepOMG.