Strange "pink stuff" freezing problem

This is the last year for "pink stuff" in my lines. I blew mine out last fall and put in what little "pink stuff" I had left from last year. I then bought another gallon and set it inside the camper planning on putting it in the lines when I had time. I never got to it and when I checked on the camper a couple of weeks ago the gallon of "pink stuff" had turned to almost solid slush and bulged the container enough that I was concerned about it breaking if I left it in the camper. According to the thermometer in the camper the lowest temp was -18.

Guy