I don't have a definitive answer for you, but I was considering this as well. Since capping is dependent upon the honey evaporating (ripening) to the point where it has ~ x% (13-17%?) moisture content, and since honey is also hygroscopic, I was hypothesizing that the relative humidity would have a rather large role in capping times. I *think* (read - have no evidence) that relative humidity would be a larger factor than cool nights... though a cool, humid night would be worst-case scenario. I think unusually cool nights could definitely be the sole cause too though.
That potentially being the case, I'm happily thinking my local climate is ideal for honey production. The bloom is (typically) very well fed by ample winter and spring rains (= LUSH!) yet it's quite dry all summer... close to a Mediterranean climate.
All things being equal (assuming same nectar source as previous years, with same moisture content), what is the RH there lately and how does it compare to past seasons (& capping times, if you have comparative data)?