They are very heavy and tipping too much ... can cause out of tipping control trouble. Otherwise, the refridgerant has no air in its system, cannot GET air in its system, nothing (like milk) inside, no other reason).
It is because there is oil in the compressor of the refrigerant system. When an appliance that is refrigerated is tilted over the oil can get pumped or just drain out of the compressor and into other parts of the system causing blockages in small lines, valves, etc. Thus this can cause catastrophic failure.