I have been watching this for the last couple weeks on Flypool. These miners very rarely show up on the Top miners list as they are not up long enough. I got a couple of screen shots that show they are mining in very short burts and once in a while are up long enough to cause a large swing in the network hash rate (they are all gone already).
My suspicion is that they are conducting testing to see how fast they can switch to ZEC from Etherium in preparation for Casper and PoS on the ETH network. Upon launch Casper will have 1 block out of 100 mined by PoS. To a big miner that is ~15min of mining time wasted each day if they do nothing and just stay on ETH. These farms are so big they would be insane to not start testing rapid switching to other coins for PoS blocks. This could have a significant impact on other networks as these miners rapidly switch into and out of networks to avoid Etherium PoS. Right now things look random but if this is in preparation for ETH PoS then they will be synchronized when Casper comes out (~1 switch very 25 min). What does that mean for how the difficulty is calculated on ZEC?