Same happens to flypool not sure if its a group of miners or single farm. They "hammer" the pool for a short times, this started like about 1-2 months ago.
I think it is a group of miners - as some are named "anonymous" and some use names on supernova. I drop to almost half the normal payout per round when they show up. Sometimes it is one or two and others a group of four or so.
If they stayed mining on the pool it would not be an issue but they only stay for between 10 - 20 mins. They do not appear to impact on the network difficulty so their hashing power is staying on the network but just on different pools.
The only way around it is to find a different pool that is not on their "circuit".