For me it's usually Violence that grows rapidly, Terrorists are rising often
(but I tend to focus on food a lot, so maybe that's the reason)
Hmmm, I'd suggest take a deep look into stuff like: policies (many bonuses come from direct boost, not per-turn; for example in Liberty), religion (some beliefs can give a lot of culture, if that civ is a founder and many cities are following that religion), resources (in CBP some resources give + culture), and if you're playing with Events & Decisions, there is a decision that reduces acquiring new policies by a significant margin (-15% IIRC).
Some other factors: policy that reduces getting new policies, if that civ is Poland (getting free policies), the civ's ratio of wide-tall empire structure.
But that more than double policy number seems fishy, especially on King.