The hard fork and the creation of 2 different coins can markedly affect the price of both BTC and BTU. Imagine that all of a sudden the coin supply doubles; this will definitely pull price downwards. Also Bitcoin Unlimited is supported by Roger Ver, a bitcoin entrepreneur that is claimed to own around 300,000 bitcoins, bitcoin.com and a large number of influential bitcoin domains. Ver can influence the popularity of BTU, via using bitcoin.com as a marketing tool, because, to the public, bitcoin.com might seem like an official bitcoin portal.
To support BTU, Ver can dump his BTC to drive its price to very low levels. Definitely, moving 300,000 BTC into the market in a short period of time can cause a disastrous price crash. Even more, Ver will not be the only one selling his BTC, as there are numerous BTU supporters who would also want to dump their BTC for the same reason. On the other side of the picture, as BTC supporters will also receive equal amounts of BTU, they might try to do the same by dumping their BTU coins.