Originally Posted by UnitedSkies
Here's one possible explanation:
Transoceanic flights often have groups booked (but not ticketed) in large numbers, and which relatively close to departure either cancel or just don't materialize.
In addition to that there are travel organizers and "consolidators" taking blocks for pushing packages with high margins. But these only explain the sudden availability in many flights of significant blocks.
What is curious is the sudden availability of seats that appear in UA/AA flights corresponding to announced fare sales while prior to the sale day, there might have been only a few seats on Y and B fares. Suddenly on the day of the sale you see availability down to S7 for selected flights on UA with the sale fares being in S bucket. It is not like UA timed the release of blocks by tour groups to design the sale
I do suspect there is plenty of inventory management (in addition to revenue management) going on.