This is a common problem. It is caused by the pressure in the cooker reducing too fast. When there is no pressure in the cooker and the jars are still above the boiling temperature they simply boil over. To keep this from happening reduce the heat to the canner slowly by ramping down the stove temperature over a longer period of time rather than suddenly turning off. Allowing more time for the pressure and heat to reduce over a longer time and for the jars to cool within the cooker over a longer period of time is probably the only way you’ll ever stop this problem. It also helps to put a few dish towels on top of the pressure cooker at the end of the process to keep the cooker from cooling off too quickly. Any leak in the pressure cooker allowing steam to escape faster will cause this problem also. Never decrease the pressure quickly and always allow as much time as possible for the cooker to cool off. Check your gaskets and make sure pressure is being held in the canner as it cools off. The pressure in the cooker will naturally decrease as it cools off without venting. Good luck.
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