I suspect it had to do with Nicehash. Nicehash has a "fallback" feature where, if your mining equipment is not currently being rented by anyone, they will rent it and try to mine blocks with it. Since they are contractually bound to pay out money to you whether someone rents your equipment or not, pointing it at their own "fallback pool" helps them earn *something* which is better than *nothing.* I suspect that, for a while, they experimented with pointing this "dormant hashrate" at Ocean instead of at their own "quasi pool," which boosted Ocean's hashrate by about 10 exahash.
But this hashrate was not there reliably (because people might rent it at any moment), which led to a situation where the hashrate contributed to Ocean by Nicehash kept dipping down and rising. Eventually I suspect they stopped their experiment and started pointing unrented hashrate at their own pool again, causing Ocean to dip by about 10 exahash.
