Not only were they [mammoths] flash frozen as you describe, but they were also asphyxiated, many being found with erect penises consistent with DEATH BY ASPHYXIATION. Their lungs were filled with sand and dirt and they were sitting on their hind legs with protruding tongues as though they were gasping for air as they died.
Something brought VACUUM and COLD right down to the SURFACE of the Earth, and as the air was sucked out from the Mammoth habitat it filled with debris which entered the lungs of not just mammoths but other creatures found the same way. Then everything was covered with frozen muck and remained until the present times. Something cataclysmic happened IN AN INSTANT.
I know that air pressure goes down as a storm approaches. Super cells have both updrafts and downdrafts. Super cells can produce up to grapefruit-sized hail. What about a storm so powerful that it literally sucks all of the air out of the local atmosphere as it advances and then flash freezes everything with super cold outflow as it passes? And, just for good measure it deposits a heavy layer of frozen muck over everything before it is gone,, dragging a severe, permanent cold front behind in its wake, leaving cold that persists for a millennium. But, what could cause anything like that? Pole reversal?