Why would a wood bench crack?

we have a wood bench at the tops of our stairs in our house. We bought this bench already embled but without any kind of treatment to the wood. We applied a varnish and then a clear coat of varnish on it. It has been in our house about 2 years and all of a sudden on the one side there is a crack down the one side. The bench is one that has storage inside and the crack goes from the top to the bottom and from outside to inside. there is never anything sitting on it or people don't even sit on it and we cannot understand why it all of a sudden would have cracked. Does anyone have any ideas or suggestions?