Why did my Galaxy S4 lock my SIM?

[BuildingASpaceship]

I woke up this morning to find my SIM card locked. I entered my PIN, and the phone accepted it, so no harm done, but why did it do it in the first place? The phone wasn't turned off, and there was plenty of battery time left both when I went to bed, and when I got up.

I got my SIM card back in 2008. For the first five years I had a Nokia phone, and it never asked me for my PIN just out of the blue like that. Less than one month ago I got a Samsung Galaxy S4, and now this happens.