I have had this happen twice now. Both times were while charging during the night.

On occasion the phone will not charge correctly during the night. It seemed that if I left the phone on the charger while trying to reboot (or power on), it had a chance of bricking and going to the autoloader screen. Both times were with different chargers and batteries. The first phone that did this worked on and off (bootloader screen, then worked fine, repeated) over the last few days before it finally died.

Now, if the phone does not charge properly in the evening I unplug it before powering on. It might not be the real reason, but I haven't had one brick on me since.

Edit: On both of these occasions, nothing was recently installed.