Comments turned off for no reason quite suddenly?

This happens to me about once every couple of months.

Today I checked my blog, and realized that commenting had been turned off for all posts I have made. For no apparent reason.

So using the bulk edit feature I turned commenting back on.

I then checked the settings and noticed that commenting had been reserved for registered users, again I turned this off since I don’t use user registration.

At the moment I suspect this is a security issue. Anyone else experience this?

My password is secure and I change it every couple weeks.