The reason for that disconnection might be one of the following:
1) You reached an idle timeout.Increase the idle timeout value.
2) Someone cleared the tunnel.check for the tunnel configuration.
3) Your SA has a very short lifetime.
check if isakmp nat-t is enabled.If not enable the same and check for the issue.
check if TunnelEstablished Registry key is set to 0 which is the right one.
Refer the following url for more info on troubleshooting VPN issue:
http://www.cisco.com/en/US/products/ps6120/products_tech_note09186a00807e0aca.shtml#isalife