Consider the market for a bond. Smart investor figured out that the default risk on the bond is lower then we thought. More people find out and there's a lot of demand for the bond so the price goes up. Now the bond trades above face value, so new investors get a lower % return, and the bond is no longer a good deal, so the price stops rising.

The same issue holds true for stocks, which are also valued based on a claim for some predicted future cash flows.