There's usually a separate switch/button to toggle the APU generator. Not uncommon for it to always be left in the on position, and pilots to get used to that, and thus it is sometimes missed that it may be in the off position, it is technically better to start the APU with the generator off so there's less load on the starter, but most pilots don't care if it's not specified by company procedure... So you see your APU running and switch off the ground power, and the plane goes dark. Embarrassing, but I don't think it creates any paperwork, so not a big deal. I'd bet that's what happened on that flight