[Cybourgeoisie]

They are likely mining Bitcoin Cash, a recent fork of Bitcoin. It is currently more profitable for miners to mine Bitcoin Cash, so a large number have opted to switch over until the profitability ratio flips back to Bitcoin.

There is more information about the relative hash rates for both chains at http://fork.lol.

[petertodd]

Worth noting that miners don't receive the coins they mine immediately - they can't be spent until they're 100 blocks old - so whether or not it is more profitable to mine on one coin or another is actually a question of the future value of the coin, not the present value. BCH has had some really rapid price swings in a matter of hours, so miners who choose to mine it are exposing themselves to those swings unless they hedge that risk.

Secondly, as transaction fees are a supply and demand system, demand for transactions on BTC (tx demand on BCH is negligible) can shift that profitability ratio simply by paying higher fees. AFAIK miners aren't setup to switch back and forth on a block-by-block basis yet, but I wouldn't be surprised if they start to do that in the future.