It’s because of a change in the solar wind associated with the downward trend of solar activity during the last few solar cycles. This downward trend will continue for another few cycles, most solar charters say.
The change results, among other things, in livening & expanding the magma and thus increasing volcanic activity. If the emissions from the increased activity (including undersea volcanoes which number many times more that the number of above-sea volcanoes) are severe enough and great enough a result can be a noticeable cooling of the earth, such as during the Little Ice Age.
What is the postulated mechanism for solar cycles influencing volcanic activity? Hard for me to see how magnetospheric effects with relatively modest field strengths can reach down through the crust to noticeably stir things up.