This is not exactly a rumor. This is news. And the price rises due to this. It is not known to what level the price of Ethereum will grow. It is possible that this is only the beginning of growth on this coin. I doubt that the price will fall much after the fork.
Now when we look at the market, what actually happens is a not so big increase. and it is true that it is possible after the fork can later fall in price
So far, it is unclear what will happen to Ethereum next, as there are just assumptions about the growth of its price or Vice versa decline. I am more inclined to believe that there will likely be a decline, as most likely after the Ethereum fork itself, many will sell.