Involuntary reboot log 8.4

Created 6/14/2016, updated 3/12/2020

This one was a bit scary. Total freeze when typing in Chrome. I figured perhaps the battery had gone flat without warning me (turns out, no, I was around 80% full). Open up machine plugged in: black screen, fans on. 15 mins later, hard reset. Machine boots up multiple times (?) and lands me in the "Reset Password" app. Reboot. White text on black screen (progress output, not a kernel panic. Worrying.

Statistics so far

Failure type: hard reset
OS version: 10.11.5
Uptime at moment of failure: unknown
Kernel panics: 1
Hard resets: 3
Total failures: 4
Start of recording keeping: 2 June 2015
Total days to date: 211 days
Average time between failures: 52.75 days

Historical data

For comparison, previous generations of machines (from most oldest to most recent) that I've used have had averages of:

22.73 days (over 1,364 days): iMac (17", Early 2006).
20.06 days (over 321 days): iMac (27", Late 2009).
19.11 days (over 707 days): MacBook Air (13", Late 2010).
41.86 days (over 293 days): MacBook Air (13", Mid-2012).
2.0 days (over 2 days): loaner laptop (MacBook, unsure of model).
27.176 days (over 462 days): MacBook Air (13", Mid-2013).
35.0 days (over 140 days): as above, but with a replacement shell.
Current machine: MacBook Pro (Retina, 15-inch, Mid 2015).