Given the BTC/BTC2X futures at 0.2 BTC2X for 1 BTC, a price drop of 20% is logic. A drop of 30% is understandable, if we assume that bitcoin price is currently overrated.
As it's not a straightforward fork as BCH was, I'm not sure I'd put too much faith in any futures prices. If you did it right you were guaranteed to get your BCH.
I wouldn't put much into the futures markets. For one thing, it really can't inform us about future demand, which will be heavily influenced by future contingencies. Which coin will have majority hash rate? We don't know. Which coin will have the "BTC" ticker on the largest US and Japanese exchanges? We don't know; that depends on miners and on Bitflyer articulating their hard fork policy more clearly.
The B2X futures contracts (e.g. on HitBTC) are wild speculation. They can go to zero. And the coin split contracts on Bitfinex only tell us that there are more ideological dumpers on the #No2x side. But that group could end up being so small vs. total supply that it doesn't matter. The low volume of all these markets suggests that we can't take them very seriously, either.
One would think that parties interested in creating the perception of a successful 2x fork would want to pump the price of the futures markets. But maybe not: as we see with Bitcoin Cash, after hitting rock bottom, the hype cycle can start once again. It may be more effective to show a rise from the bottom to paint the perception of 2x "overtaking" Bitcoin.