James Montier of GMO just published a piece about what he calls the "cynical bubble". Professional investors see the market as overvalued but have no choice but buy in. These "fully-invested bears" contribute to a "near rational" bubble (current good fundamentals extrapolated), something Montier calls a "greater fool market" in which the participants believe they can exit first when the market turns.
Full piece: The Advent of a Cynical Bubble
The "near rational" bubble:
Professional investors think the market is overvalued:
But they are also fully invested in the market: