In a typical month, 15-25% of the cars sold are from BYD.
If there is only a single distributor, such moves are likely because the distributor stocked up enough COE from earlier bids or open-cat COEs.
However, BYD has a few dealers.
It appeared to be an coordinated action by BYD dealers to bring COE price down. Doing so, they can either price their year-end cars competitively or trick competitors to bring down their guaranteed-coe prices (and lose money when COE rebounds strongly)
If this hypothesis is true, then the latest guaranteed-coe car prices will not drop more $12,000 and $16,000 for CAT & B respectively.