First of all this is not a brand new theory, I recently read a whole book on the volcanic activity in Europe as the Eurasian ice sheet melted at the end of the last ice age. Incidentally, the Pinatubo eruption had nothing to do with ice. However, it is very clear that the increased vulcanism in Europe at the end of the last ice age was probably triggered by the uplift of the land as the ice melted, rather than being connected to the beginning of an ice age. The Lacher Zee (sp?, can’t find my book) volcano in Germany, and volcanic activity in France occurred at that time, and there were also some very massive eruptions of Vesuvius, far larger than in historic times. These areas are on the north side of the plate for the movement of Africa.
There are no recent volcanic areas in the center of continental North America, however, there are some really old volcanic deposits (north and east?) near Yellowknife in Canada with Kimberlite deposits where diamonds have been discovered and I believe are being exploited now.
While there may be some effect from isostatic rebound, there is a chicken/egg question. Volcanic activity will be preceded by higher heat flow at the surface. Once started, ash deposits will decrease the albedo of existing ice/snow fields and accelerate melting, provided atmospherically suspended ash is not significant enough to retard insolation. Certainly, melting will accompany volcanic activity. To say melting owuld cause volcanic activity is a far stretch when there is the entire Canadian Shield in isostatic rebound from the last Ice Age and recent non-margin volcanic activity is pretty small.
The kimberlites in AB, for example, are 45 to 75 million years old, whereas the Caldera activity in Yellowstone is a few hundred thousand years old, and the Long Valley Caldera only a few hundred.
If melting ice caused volcanoes, (instead of the other way around) Saskatchewan, Manitoba, North Dakota, and Minnesota should be prime for eruptions--the ice sheets here were well over a kilometer thick, and in many places three times that.
Where melting ice and volcanoes are associated at all, my money is on heat flow from the magma melting the ice, not the melting ice causing volcanism.
There is actual volcanic debris along I-40 in the interior of California also. I saw it myself.
Please keep in mind that the evidence of volcanic activity which settled on the ice sheets washed away when they melted. The data subset may be skewed by the lack of preservation of the pyroclastic sediments. The apparent increase may be a question of preservation of ash beds, with those after the ice melted being preserved and those which fell on the ice sheets not.