The Azure SQL Database service triggers an automatic failover after a failure is detected and the grace period has expired. What is the grace period? You can find this setting when building a failover group.
Quite an interesting situation I found myself in where I was perplexed for about 5 minutes. I was connected to an Azure SQL Database where I was configuring some users where then I executed a query and was presented with the following message:
Msg 3906, Level 16, State 2, Line 2
Failed to update database “testdb” because the database is read-only.
It is always a good idea to test your failover processes when you have setup failover groups in Azure. I have the following setup:
All About Data 