...a shrinking ice cap reduces the pressure on the earth's mantle, causing it to melt and creating magma. Also, this frees tectonic plates up to move against each other and cause the friction needed to initiate earthquakes. This tallies with mathematical models that suggest such processes may potentially lead to more earthquakes, tsunamis and volcanic eruptions.
So when ice caps and glaciers melt, we get more magma and the tectonic plates are free to get squirrelly. I'm no expert, but I could buy it.
The more important point, though, is that even the experts don't know what kinds of feedback loops and side effects climate change might trigger and they won't all be nice surprises.