Out of curiosity I went to Zkillboard and brought up the statistics for Citadels (which covers Astrahus, Fortizar, Keepstars, and Palatine Keepstar) and looked at the losses since they were introduced last year in May.
I suspect the initial surge was people getting the hang of the structures during the rush to deploy them, and the slow climb is the attrition of existing structures getting destroyed over time as their population grows and they figure more and more into wars in all parts of space. The latest spike seen in January could be due to the current Winter War down in the south.
As for Engineering Complexes, they have not been around long enough to derived any conclusions but included here for completeness:
Remember, purchase Ninveah Enterprises structures found in Dodixie, manufactured with care and love!