I've experienced something similar. I'm not 100% sure if it's the same phenomenon you're describing, but I suspect so. It happens if you use coffee that's ground too finely, and doesn't have to do with boiling. What happens is that the fine coffee grains block all the holes in the mesh. This means that the water is under more pressure than usual, since it can no longer pass through the plunger. Because of this the water ends up escaping by forcing a small part of the mesh away from the side of the carafe and squirting out at high velocity. The reason for the high speed is just that it's passing through a small aperture - it's the same effect as when you put your finger over a hose.
To prevent this from happening, you could try a coarser grind, or if you already use coarse-ground coffee, try pressing even more gently on the plunger. If you meet resistance then try lifting the plunger slightly before continuing - this should redistribute the grounds slightly and hopefully unblock the holes in the mesh.
From a physics point of view it's worth mentioning that boiling would be a very unlikely response to compressing hot coffee. It is possible for a liquid to be in a "superheated" state, where it's above its boiling point yet remains liquid. When water is in such a state it can indeed boil very suddenly. But this state can only be reached if there are no nucleation sites available to allow steam bubbles to form, and the coffee grounds would probably provide excellent nucleation sites, so if the water were superheated it would boil as soon as you poured it onto the grounds. (This would probably produce very bitter coffee.) Liquids can also suddenly boil if their boiling point decreases below their temperature - but pressing the plunger increases the pressure, which increases rather than decreases the boiling point. This is true for all liquids (by Le Chatelier's principle), so we would never expect boiling to result from an increase in pressure.
