The last major forks (ETH:ETC, BTC:BCH) both created net positive value; that is, the total value held after the split was greater than the value of a single chain.
So, I think the so-called ‘smart’ money ahead of the fork has been to buy BTC, since history seems to show it will be worth more post-fork. Fork looks more likely -> more buying.
And of course the rising price creates its own momentum. There are three general outcomes from the fork: one or the other side ‘wins’, and there’s a shit-chain spawned, or they both coexist.
In the coexist world, we have a pretty interesting set of market moves ahead; if the debate is as divisive as it looks on twitter, then the whole crypto world will be holding ‘shit coins’, but not ones others think are shit, along with their preferred fork.
It seems possible that they may turn some of those into Ethereum, and it seems likely to me that some traders will ‘get back’ to Ethereum post-fork; all those would indicate some cautious optimism about ETH prices later this fall.
And of course if BTC just totally nukes itself, then I think you’d see a major flippening back to ETH.