Sometimes this is normal, expected behavior: when there is a problem with the Internet connection, for example. Other times it can be because of a misconfiguration in the configuration files that control the connection.
Please follow the instructions at
Read Before You Post and send the "diagnostic info" in a reply. In following the instructions, please connect the VPN, wait for the problem to happen at least twice (please note the times when the disconnect occurs), disconnect the VPN, and then collect the diagnostic info. Note the times that the problem occurred in your reply (that will help us locate the problem in the logs).