1) everything runs fine and bitcoin price will continue to vary sideways.
2) the hard fork causes problems and BTC price plummets down to zero
So you see it's either sideways or rock bottom! There's no possible scenario where the hard fork will pump BTC price. It's not like people will think "omg block size increased, let's all buy bitcoin!".
So if you sell now, you're protected from a rock bottom price drop.
If the transition runs smoothly, you can buy back in later.
Sell now to protect you against the remote possibility of the fork ruining bitcoin.
Have they already agreed about a date at which the hardfork will be executed?