What causes prices to go up? What causes them to go down? Obviously, no one knows for certain when it would happened, but has there been a consistent "pattern" lately? When prices have gone down, have you known to a reasonable degree than they would bounce back up? How?
This much I can answer for you. Price goes up when the demand for Bitcoin increases in the market or there are more investors in the market because of moves like Paypal allowing the purchase of bitcoins. The price drops when the demand decreases while the supply is increasing which is rare actually because the bitcoins are limited and supply is always going to shrink slowly but surely.
The only reason BTC might ever fall off would be is some other coin matches and surpasses Bitcoin in terms of popularity and use cases.