What natural processes can cause Catastrophic Failure of a Hard Drive?
First, MTBF is Mean Time Before Failure. It's the average time before any failure that causes data-corruption at all.

There are lots of things that can cause permanent data corruption, or a damaged drive. 

If the controller PCB on a hard drive experiences catastrophic failure, the "drive" as a unit would effectively have "completely" failed, though it's theoretically possible to transplant a controller from a different drive of the same model and possibly recover some of the data.

Alternatively, if you have a read head hit the platter surface, it can become damaged and being scraping the disk surface off the platters. While this isn't "instantaneous", it could happen fast enough that even if you de-power the drive, just the momentum of the disks would be enough to generate sufficient particulate matter that the drive is no longer functional.

Simply dropping a drive when it's running can easily be enough to trigger this kind of failure. 

Frankly, most mechanical failures for hard drives are going to be somewhat catastrophic, simply because the interior conditions of a hard drive are so delicate. Anything that releases particulate matter into the drive internals will likely cause irrevocable damage, simply because it will get dragged between the heads and the platter, and damage one or both of them.

The fact that modern drives are so amazingly reliable, and often do fail in a predictable (and gradual) manner is really a testament to the amazing design and engineering effort that goes into them.