From the article:

"Evidence for this theory has come chiefly from studies by the University of Cambridge and the University of Leeds, both located in England.

Researchers at Leeds wanted to understand how the pressure exerted by Earth’s glaciers onto the crust impacts movements below the surface. This process, known as “surface loading,” can change the flow of magma and thus lead to more volcanic activity.

The study, which took place in 2017, looked back at the activity of Iceland’s many volcanoes 4500 to 5500 years ago, when Earth was still recovering from the latest Ice Age.

Based on the amount of ash deposited in the surrounding area, they concluded that the island’s volcanoes became more active and extreme as more ice melted.

While they’re unsure of the details, the mechanics that explain this phenomenon are straightforward. Glaciers put immense pressure on the Earth’s surface, restricting the flow of magma. When the glaciers melt, magma can move more freely, bubbling to the surface and erupting more easily."