BTC price goes down and S5 price goes down as well??? This is new.
I think price is slowly being adjusted to the fact halving day is less than year ahead which will cause S5 contract being virtually worthless. Current ROI takes too long at this rate and it is quite clear after 260 days it won'tbe possible to sell S5 at reasonable level. I think if you already ROIed selling all hashes now would yield the same income as keep mining and selling later.
You forget one important aspect, BTC price. According to your theory, BTC price ll stay constant after halving and this ll make S5 worthless. I can assure you, this is NOT going to happen. Look at LTC, couple months before halving, the price went up 3x. If something similar happens to BTC, S5 will mine less BTC, but at the same time that BTC ll be valued more.
This is the only option. If this does not happen, half of the farms will close down and there will be no BTC to satisfy demand and the price ll go up again to compensate.
I think recent BTC and S5 price movements are related to this BTC XT issues as well as naive belive ETH will be the next crypto gold. So many people sold their crypto assets to buy ETH. The result is not what they have expected, at least not yet.