Interesting, as I get a denounce request in the significant majority of my games. Some games, it will even happen more than once.
Are you playing at Deity? Do you accept DOFs?
Typically I will have DOF with both AIs, and the first AI asks me to denounce the second AI (which would earn me the backstab penalty). I decline, and more often than not, the first AI then immediately denounces me. If I do not have a DOF going with the second AI, then I almost always denounce. But it is unusual to get the denounce request when I do not have the DOF going with second AI.