1. Poorly written code in the kernel module crashed the whole OS, and kept trying to parse the corrupted files, causing a boot loop. Instead of handling the error gracefully and deleting/marking the files as corrupt.
2. Either the corrupted files slipped through internal testing, or there is no internal testing.
3. Individual settings for when to apply such updates were apparently ignored. It's unclear whether this was a glitch or standard practice. Either way I consider it a bug(it's just a matter of whether it's a software bug or a bug in their procedures).
4. This was pushed out everywhere simultaneously instead of staggered to limit any potential damage.
5. Whatever caused the corruption in the first place, which is anyone's guess.