Two mining pools (Discus Fish and Antpool) were engaged in what's called "SPV Mining", where they assume a block broadcast by another miner is valid based solel

Two mining pools (Discus Fish and Antpool) were engaged in what's called "SPV Mining", where they assume a block broadcast by another miner is valid based solely upon the block header (essentially, just checking that the block was "difficult enough"), without verifying (or downloading!) the transactions in that block. This strategy lowers their orphan rate at the cost of the risk that they'd waste work by mining on an invalid chain. This low risk was increased by the BIP66 enforcement (activation happened once only 75% of the blocks had the version 3 "flag").