One root cause I've seen over during database outages is insufficient IO. It's caused by the other most common root cause: lack of actual expertise. Until you've been through the wringer, it's easy to lack respect for data: how important it is, how fucking large it is, and how long it takes to do basically anything with a few TB of it. If you didn't hire a real expert, and your DB-for-a-day guy miscalculated the spec for your bare metal clusters, you'll find it extremely hard to magic up faster hardware during a 4am full-outage when Europe wakes up. If you're on cloud, at least you have the flexibility to increase block storage IOPS or quickly reboot a node on beefier hardware or faster network connections. But suddenly the "cheap" DIY database is a lot more expensive than you budgeted for, and the phrase "nobody needs a dedicated DB guy" starts sounding really short sighted.