Windows Phone reset refused to start, then hard reset itself

[mj0]

I took out my Lumia 830 running Windows Phone 8.1 Update 1 after it was safely stored away for about eight months. I hadn't used it at all during this time (and I can definitely rule out any possibility of anyone else doing so), and took out the battery before I put it away. Today, after putting the battery back in and turning the phone back on it refused to boot at first. It was stuck at the Nokia logo and didn't get any further. The logo then disappeared after about 15 minutes and the screen went black but the phone didn't react to anything - camera button, power button, nothing. I did a soft reset (power + volume down) after which it restarted and booted (which took around 5 minutes), this time getting stuck at the Windows logo that appears right after the Nokia logo. When it finally managed to boot my home screen was empty except for one tile - data optimization. Needless to say it most certainly wasn't empty before I put it away, and I have no memory of ever using a tile/program called data optimization.

This time I was able to turn the phone off using the power button, then turn it back on and now it booted into the initial setup routine as if I had done a hard reset. I've managed to get through the setup process and am now finally able to use the phone, albeit it's factory reset so all my previous settings, apps, messages, photos, etc. is lost. I can safely reboot it as often as I deem fashionable and it reboots within a minute, as it did when I first put it away eight months ago.

Any idea what might have happened here? I was expecting to resume normal operation after putting the battery back in but the phone seems to think otherwise.

 

[mj0]

The phone now updated to 8.1 Update 2, I had to install two phone updates back to back and it's now up to date.
Still I'm wondering what exactly might have happened here. Somehow I don't trust this phone anymore...