I think they were taking advantage of the other miners trying to include as many tx in a block to use their hash to quickly find a block. This happens occasionally day-to-day
But naturally this should not have been happening during the stress test for a major miner as many transactions were in their mempools - they should have put in as many tx as possible
OTOH, stress test was the best time, it proves that mempool can be processed even if small block miners are around. A great argument for no-limit blocks.
