13" non-Retina MacBook Pro locked up again

It happened before a couple of years ago, although it was slightly different. Back then I plugged in what I knew was not an MFi Lightning cable and everything just went dead in a split second. The charge light started flickering green/orange before it went completely dark. Nothing worked. The battery status button wouldn't do anything. Couldn't power up, power down, even trying SMC reset. I could hear a faint hum coming from the board, although the hard drive wasn't spinning and the fan stopped immediately. I took it to an Apple Store without an appointment and was told that I could come back in the morning to get on the non-reservation waiting list. I gave up, looked up how the memory or drive are replaced, and followed the instructions for how to disconnect the battery connector. When I reconnected it I could power it up and it never happened again.

Until today when something similar happened. This time I plugged in a MFi Lightning cable with my iPad Mini 4 attached and everything just went dark. I've got an SSD now so it wasn't like the previous time where I could tell the drive was no longer spinning. I didn't hear any noise, but my first impression was that I should take off the bottom cover and disconnect the power again. And this time it was pretty warm and I could kind of smell it, if anyone here is familiar with what a circuit board smells like when it gets hot. I pulled off the battery connector and let it cool down for a few minutes before I reconnected. And it fired up immediately with the power button.

So - any idea of what happened? I'm familiar with certain electronic issues like latch-up, but I'm not sure if that happened here. I've described this as somewhat of an intermediate state where it can't power down and can't power up unless power is completely removed. I've heard about users with similar conditions, and some would just wait for the battery to die, charge it, then it would recover. There seem to be a few people reporting similar conditions on the newer MBPs, but those of course aren't as user-serviceable as a Unibody MBP.