As James Surowiecki makes clear in his book "The Wisdom Of Crowds", the wisdom is only apparent in situations where the people are not able to talk to each other. But real estate agents are able to talk to each other.

In Surowiecki's telling, if you get a room full of people to write down a guess about how many jelly beans are in a jar, the average of all the guesses will be surprisingly accurate. But if the people are allowed to yell the answers aloud, the first person who yells a guess "anchors" the guesses, and so if that first guess is radically wrong, a substantial bias is introduced into the overall distribution of guesses.

Real estate agents in a small geographic area, such as one city, often talk to each other, and therefore they generate a conventional wisdom, but each of their opinions influences everyone else's opinion. So you can get a herd action that is greatly at odds with reality.