Q: 5s suddenly re-boots, then asks for activation...(re-activation, basically)

I was in the middle of a phone call (phone call was obviously dropped) when my 5s suddenly re-boots and then asks for me to activate it. I had no other option except to enter my apple user id and password and then the phone went back into a normal state like that never even happened. I have had the 5s for a few years now and its the only 5s I have had.

I have pre-ordered a 7 Plus that is due to for delivery sometime in November.

Why did my phone do this? Some screw-up to do with pre-order?