Blender Physics

Got a kitchen physics question for you. I have observed that when I run my blender to mix a hot drink, the water leaps farther up the vessel. If the water is freshly boiled, it’s likely to spray hot water out the little vents in the cover. Coffee that has sat in a French press for five minutes after boiling almost never does this. Room temperature or cold ingredients never do it.

So, what’s happening here to make the difference? Is it steam pressure from the near boiling water that makes the difference? The hot liquid warming and pressurizing the air in the vessel? Or is it some other interaction between heat energy and the sudden torque the blender applies to the fluid?

Note it’s a 2 liter Kitchenaid blender vessel, and this happens with 0.5L of boiling water in it (or more.)