Hard to say exactly, but perhaps someone with large holdings in an already established crypto-currency might have motive to want Ethereum crushed, or at least be perceived with less legitimacy than whatever other coin they hold (likely BTC)

Off the top of my head the people with the largest long-term vested interest in bitcoin would be 1. Satoshi Nakamoto (whoever s/he really is) 2. Winkelvoss twins 3. Whoever stole coins from the various exchange hacks