I'm not sure what's occurred because I wasn't there, but I suspect what happened may have been the result of super heating and has nothing to do with pressure. Sometimes, if a liquid is placed in a very clean container something called bumping can occur. The hot liquid finds nothing to form bubbles on (to nucleate). This occurs in chem labs often. A liquid is heated to the boiling point and continues to get hotter without bubbles forming. Then, if the liquid is shaken it will suddenly all boil at once and often practically explodes. The same thing can occur when freezing. Water can be cooled below the freezing point without ice crystals forming. Then, if a piece of dust or an ice crystal is added to the container, the water will almost instantly all freeze. I suggest that, as one of your answerers suggested, you gently shake the pot after you've released the pressure and before you remove the lid. If this is the cause, it's a very rare occurrence and has never happened to me in home cooking. It's happened in my lab often.