A. The most common reasons for loss of liquid are packing jars too full, packing food too tightly into jars.
In a pressure canner - as above and or fluctuating pressure, or lowering pressure too suddenly.
Also, if all air bubbles are not removed from jars before processing, the liquid may be lower in jars after processing.
This poses another question -
Q. Should liquid lost during processing be replaced?
A. No, never open a jar and refill with liquid (this would let in bacteria and you would need to process again). Loss of liquid does not cause food to spoil, though food above the liquid may darken.
The above statement can be confusing however if the jar has sealed and there is a little food remaining outside of the liquid due to liquid loss during processing and not due to spoilage then it is generally safe. I usually just discard the top portion of the jar contents myself due to asthetic reasons although some people just stir it in and use the whole of the jar contents.
If on the other hand you suspect there may be food spoilage, then check the jars contents carefully before you use it and follow the recommended guidelines for handling spoiled food.