Initially as the temperature rises, the thermometer has a safety feature to prevent damage. Look closely at the top of the liquid column and you'll see there is a large empty bulb. With moderate over-temperature the liquid merely expands and fills this space. When cooled the liquid can simply shrink back down.
With substantial overtemperature, though, that is not enough. The liquid will start to generate a vapor pressure inside the closed space, and that pressure will rise a lot at high temperature. I'm not sure whether it is possible for all of the liquid to be converted into a high-pressure gas, but I doubt it. I expect there would be an equilibrium established between gas at VERY high pressure and some liquid. Whichever the case, the point is that the internal pressure would be very high, and the result will depend on details of the glass tube. Most such thermometers, I am sure, are NOT built to withstand such pressure and the glass will break, probably sending a few glass pieces flying off in random directions.
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