A classic case of correlated failures. If you have multiple redundant systems, but anything that damages one system is also likely going to damage others, then a failure is much more probable than you would expect based on the level of redundancy.

In this case the redundant systems were the watertight compartments. The engineers thought the ship wouldn't sink because it was unlikely for five of them to fail at the same time. But they forgot just how likely it was that any incident (e.g. an iceberg collision) that damaged one compartment would likely also damage many others.