Need help understanding a power loss event today

I'm hoping somebody can make sense of a power outage issue I had this morning.

I woke up this morning just before 7:00 to the sound of my APC battery backup beeping, signalling a power outage. I was just on the phone with APC yesterday to discuss my backup, as it is over 4 years old, and at 100% charge it shows less than 1 minute available on battery backup (APC told me the battery is gone and needs to be replaced due to age). I also get random power outages in my area. I had one 2 days ago for just a few minutes, and my neighbors (but not me) had one yesterday afternoon for a while.

I freaked out this morning because my PC was in the middle of doing a full format on a brand new drive when the power went out. When I awoke, the PC was already off, since the APC battery is bad. I turned the unit back on (assuming it was off), and it appeared to be starting up like normal. After about 15 seconds, the PC shut off again instantly. I didn't know what was going on, but I pushed the power button again, and this time, it appeared to be starting up again. Within 30 seconds or so, the PC was fully up and running, with the "disk management" window open, and formatting was continuing, like nothing had happened. I was very confused, and decided to cancel the format, thinking maybe the interruption may have harmed the formatting process. I then deleted the partition back to unallocated, and re-initialized the drive and started a fresh full format. From there, everything went fine, and the format completed about an hour ago.

Here is what was in my event viewer:

Application
-----------

6:55:41
Battery Backup transferred to battery power due to a blackout.

6:55:46
Battery backup transferred to AC utility power.

6:55:53
PowerChute causing PC to hibernate.

6:56:01
Battery backup transferred to AC utility power.





System
--------

6:56:06 AM
The system is entering sleep.
Sleep reason: Application API

7:00:17
The system has resumed form sleep.

So what happened? I thought the APC Powerchute software was supposed to put the PC into shutdown mode when there's a power loss? That's what it tried to do the other day when I lost power, but the battery died in less than a minute, before the PC had time to finish shutdown. So then why did the PC seemingly go into sleep mode, and hibernation? And why did the PC come back up so quickly with "disk management" running? My PC takes a long time to boot these days. Very confused.