I have been following Ethereum price from $8 to $400 this year. Ever since the beginning of this year, there are many ICO based on Ethereum's platform which means you must first buy Ether then purchase their ICO. Every time I see an Ether based ICO launches, Ether prices dropped after few days.
My theory is they need dollars(USD) to pay for most expenses such as hiring engineers, rents, and other expenses, therefore, they need to sell Ethers for USD. Crypto currencies have not reached to the main stream usage yet, therefore, they need to convert back to USD so more selling pressure will continue as more Ether based ICO launched.
I hope I am wrong cause I really believe in Ether's future.