Think about it, a hard fork can influence the BTC price in only two directions:
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.
We also had little panic when that hard fork happened during 2013, but then people realized that a hard fork will not hurt existing coins, only out going transactions. And devs have learned from the previous fork, they will handle it very smoothly so that almost no one feel the difference
And the fork might only happen when the current block limit is approaching and there are lots of transactions never get included due to forever increasing queue