AlwaysOn Failover - SQL Server 2014
The cluster log, SQL Server Error Log and the application log in Event Viewer should give you the reason. 

It sounds like quorum is not setup properly. Without a 3rd resource to act as the witness, a network glitch will cause a failover. 

Setup a witness, such as a file share on another server, or a witness disk. Some people use a file share on a file server or an AD server. I've also used the witness disk approach where the disk is a shared drive, like what you'd have on a Failover Cluster.