I was Babylon, playing on Monarch. I had had a war with the Aztecs and Persians, but that was probably 40 turns or more earlier and my society had been largely rebuilt. One stupid Persian unit plods into my territory, so, since I'm one turn away from Motorized Transport, I demand his withdrawal, and the Persians declare war. I kick that units butt, and redeploy my units to my borders. Naturally, during the AI's turn, ALL FIVE remaining civs declare war on me and sign ROP's. Persia sends about 10 cavalry through America to hit me at my depleted inner cities, and takes 2 cities on their first turn, destroying one.
War weariness was so bad after that one turn that I had to revolt and go to communism. My supply lines weren't cut, but I did lose one spice, but even that wouldn't have accounted for the massive increase in unhappiness from the previous turn, especially since I had Universal Suffrage and Police Stations in every city.
I think losing cities is a major factor in rising WW, as is the number of civs declaring war on you. On the flip side, it seems to me like capturing cities DOES help WW, especially if it is from the civ that started the war.