AOAG Database went into NOT SYNC\SUSPECT MODE
This situation is caused by the transaction logs not being properly hardened on one of your secondary replicas. This could happen is one of them was offline for a period of time, or was unable to connect correctly. 

To help identify the replica having the problem you can query sys.dm_hadr_database_replica_states, in particular pay attention to the log_send_queue_size column. 

There are some odd things that happen with AGs sometimes, particulary after a machine has been restarted, whereby one of the databases in the AG will just stop syncing (and there will be no logged errors or issues). Rather than remove it from the AG entirely, and then add it back in you can attempt to restart the hadr_endpoint on the secondary that is being the problem, and then watch to see if traffic starts flowing again.