Guild level decays over time. So long as the total earning rate of people within the guild exceeds the decay rate, the guild level will slowly rise.
People haven't been on much lately what with one thing and another (holidays, deadlines, postings abroad, etc), so the guild's total earning rate has dropped below equilibrium for the first time in a long while and we've lost a few levels. It should pick up again once the season's over.
The decay rate and earning rate are also influenced by the size of the guild and the current guild level, so that level 100 is hard to reach and maintain, and small guilds are not penalised too much for not having as many renown-earners online each week as the big guilds.