The USAF HF-GCS system has been quite busy lately. Lots of one time pad alphanumeric messages being sent. I haven't heard SKYKING in a little while though. Lots of obviously randomly-assigned callsigns for the station giving the broadcast.
I don't really listen to the HF-GCS, I find it pretty boring most of the time. However, I do record it 24/7 just in case something good happens, I can always go back and look at the recordings to capture good stuff. Your comments made me run a few recordings to see what was going on about the time you posted.
You posted this on 15 Nov, 2018. I assume your statement of "USAF HF-GCS system has been quite busy lately" was caused by the exercise that was on-going the week previous. The 4th to the 9th of Nov was a very busy period for HF-GCS traffic.
As for SKYKINGS, there is seldom more than 2 or 3 days between them, and often many per day. It looks like in the period just before you posted this (and back to the beginning of the month) there was at least one SKYKING (and on several days multiples) on the 2nd, 5th, 6th, 9th, 12th, 15th, and so far 2 today (the 17th).
T!