It might because you started a war with another nation and your government was a democracy or republic and your citizens became war weary. The same thing happened to me due to a mutual defense pact. Hell I was not even given the chance to decide not to declare I was forced into it due to the pact.
After about 50 turns my nation of over 50 cities with 40 having “love days” for me every day for years

went into rioting! My science went down to 0% but that was not enough to stop the ruin of my empire. I could not take my luxuries beyond 50% because of costs.
Did you draft as well? I made the mistake of doing so with 15 units! The Aztecs rushed me with over 30 vets and elite cavalry - It was not a pretty picture! By the way that was only the first wave!!