The rule of thumb says the percentage of stocks one should own equals 100 minus your age. This CNNMoney.com article discusses this topic with some reasons why people may want to be more conservative:
in retirement especially you want to be mindful of "black swan" risk. [...] it's the chance that your portfolio might be decimated by a rare but catastrophic market meltdown. We tend to gloss over cataclysmic events in our planning because they're considered extremely low probability events. But recent research suggests they may pop up more frequently than we've believed.