iPhone Died. Activation Lock Self-Engaged When Turned Back On

My iPhone died earlier tonight. I thought nothing of it until I plugged it in to charge. Instead of the normal lock screen coming up when the phone turned on, it was asking me to choose a Wi-Fi network to activate the "new iPhone." I tried turning it off and on several times, but every time the phone just went to this Activation screen. Couldn't bypass it by trying to get to the Control Center or Notification Center, as those didn't do anything. Hitting the Home button brought up two options: "Emergency Call" or "Choose Wi-Fi Network." When the phone died I had been deleting alerts from my Notification Center, so it's not like I was trying to restore the phone or erase everything. Couldn't take a screenshot or access the camera.

I was worried about losing anything on the phone that wasn't backed up, or that if I followed through with the Activation Screen prompts that I would have a wiped iPhone like the day I got it. But I noticed that when I turned the phone off it was showing my normal lock-screen background behind the blurring that occurs underneath the "slide to power off" button. So I wasn't worried about losing anything on the phone, but I still didn't know what was going on with the Activation Screen.

When I got home, the phone remembered my Wi-Fi network, so that was a good sign. But it was still showing the Activation Screen and said there was an Activation Lock on the phone. Let me compare what I saw to what this page -- Find My iPhone Activation Lock - Apple Support -- describes: that link describes the Activation Lock put on a phone when you engage Find My iPhone. The Activation Lock has a little message saying "this device was lost or stolen..."

In my case, the Activation Lock said nothing about the device possibly being lost or stolen (who would have engaged Find My iPhone to put the lock on anyway? not I) but just that I needed to enter my log-in credentials from when I set up the phone. At this point I was just hoping entering my iCloud log-in would end this annoyance, which it did.

SUMMARY: So, has anyone else experienced this? My phone's battery died as it has so many times before. Find My iPhone wasn't engaged by anyone (that I know of), and yet when I turned the phone back on it had an Activation Lock engaged. The Activation Lock said I needed to enter my Apple log-in info but didn't say why.