Why would one expect that? The demand is increasing, and given the fact that the supply is increasing at a much slower rate, then it's quite normal for the value to increase.

That being said, I don't know what caused the "sudden" (few weeks) increase from $14 to $18, as I don't read about Bitcoin that often. But I do expect the value to keep rising as more companies start adopting Bitcoin payments and more people then get interested in it. And I also expect Bitcoin to remain relatively volatile at least until its market value is counted in billions or tens of billions of dollars, when each individual transaction will have a much smaller impact on its value, in either direction.