I find from the times I'm watching ExpertFlyer for various reasons that availability does all sorts of bizarre things in the three days leading up to departure. It's interesting to see some theories/explanations for why that is. (In particular, the lower fare buckets almost always seem to open suddenly. Perhaps that's because, as someone says, they release some seats in higher cabins for those booking codes to facilitate irrops etc if they expect not to sell them anyway.)