As far as I know it is still an open question. This kind of rate change is referred to as a “decadal variation”, because the changes last ~decade(s). They seem

As far as I know it is still an open question. This kind of rate change is referred to as a “decadal variation”, because the changes last ~decade(s). They seem to be due to activity in the earth’s mantle, but it is all very poorly understood. I wrote about this three years ago: https://dotat.at/@/2020-11-13-leap-second-hiatus.html https://dotat.at/@/2020-11-13-leap-second-hiatus.html