Four of them happened hundreds of millions of years ago. The fifth and most recent one is called the Pleistocene era. The Pleistocene began around 2 million years ago and consisted of several glacial and interglacial periods. The Wisconsinan glaciation began around 100,000 years ago and ended about 10,000 years ago. There are some that say the Pleistocene did not end with the melting of the Wisconsinan glaciers, but that we are now just in an interglacial period.
Increased cloudiness is one possibility.
If the world becomes cloudier for a number of years, it would let less solar radiation reach the Earth's surface. The properties and interactions of clouds with the global climate are just beginning to be understood, but it is certain that their coverage and distribution can have a great impact on changing the climate. A nother factor that can cause the climate to cool is the eruption of a volcano. While the ash is up there, it can circle the globe and reduce the amount of solar radiation making it through to the troposphere and surface. This winds up lowering the temperature of the lower atmosphere.