Did/does your standby replica in another AZ have any instance notifications stating there is a failure? The outage report claims there were just EBS problems in only one AZ.
Multi-AZ RDS does synchronous replication to the standby instance -- I'm guessing something broke in there. Hopefully AWS will update with a post mortem as they usually do. Lots of frustrated MultiAZ RDS customers on their forums.
Maybe there was something bad with your replication server before the outage? It's hard to guess without knowing exactly what was happening at the time...
The whole point is to protect you from problems in one AZ by keeping a hot standby in another AZ. It doesn't matter whether it's due to EBS, power, etc. This is one of the primary reason to use RDS instead of running MySQL yourself on an instance.