Write latency elevated without any obvious cause

We are running MySQL on a db.r3.4xlarge with 4000 provisioned IOPs on SSD. This morning we started seeing increased write latency for long periods, without any apparent increased load to explain it. CPU was under 12% and write IOPs were below 500, and actually went down to around 100 once the average write latency increased from its normal value of around 2 milliseconds up to 150 milliseconds. It almost looked as if something were throttling the writes, despite the fact that we're paying for provisioned capacity and staying well below that limit.

After thorough investigation of our queries, we concluded that it might be caused by hardware degradation of some kind. Since it's a Multi-AZ instance, we rebooted with failover and the problem went away, but then returned a little over an hour later. If this is something we're causing, we'd like to know how to avoid it, but we haven't seen it before in the three years we've been running this instance. Does anyone know what might cause write latency to be increased? It would also be good to understand more about how write latency is measured if that information is available.