That is the problem with the "CBHC is 200% safe – all bricks are user error!!11!"-attitude that pops up uncalled whenever another Wii U bites the dust.
Humans make mistakes. Humans forget things.
If you're familiar with CBHC it is completely clear: Autobooting means the cold boot thing is active and the rules for prevent bricking apply. If not (bought used console, modded a few years ago following some guide and never really cared for afterwards, friend did the modding…) a splash saying "Autobooting…" is just a splash saying "Autobooting…". Not everybody is completely keen on things like that. Some people try things out without reading the manual (big mistake of course). Half-hearted activity – maybe with the only (and bad) goal of playing illegal copies on a console – followed by losing interest
Knowing my own limitations and shortcomings and blunders I can understand how things like that happen. Having played around with Android phones lately while being overwhelmed by the never ending mass of information I can really understand that people forget about important facts like the CBHC rules.
This is why software should be as fool-proof as possible… and CBHC is not fool-proof. I know people who I would not dare to give a modded 3DS. They would surely use the GodMode9 hex-editor on nand.bin. Not even remotely thinking about giving them a Wii U with CBHC (or a phone with TWRP)
And yes, I'm using regular, non-cold boot Haxchi although I know the rules.