Once a client (FileMaker Pro/Go) connects with FileMaker Server 13, then FileMaker Server 13 continues to ping the client over the network (Ethernet/Wi-Fi) to make sure there is still a connection. If FileMaker Server 13 is unable to contact the Server, it will try to connect for two minutes, and if no connection can be established, then the connection to the client is dropped.
Is it possible your iPad goes to sleep? Are you accessing any other application? Is FileMaker Go idle for an extended period? What is fmreauthenticate privilege set for?
Any other information you can provide may help narrow the number of causes.