Either a malicious or ignorant Nicehasher pointing too much at them at once. We've got one of our devs working with them to sort out why their pool keeps splitting the chain when Nicehash gets pointed at them but not any of the other pools.

Good opportunity to further show why it's important to spread hash power around pools. All it does is screw over the miners and the nicehasher on BSOD while they orphan the split chain blocks and reindex.