I used to have a vacuum sealer.
Something to remember is: Water *expands* when it freezes (think ice cube trays).
If you have a frozen item with a high water content that has been vacuum sealed and then you heat it up: it is going to shrink.
Shrinking in a negative pressure compartment will steadily add pressure until something 'gives' and the pressure is equalized again.
The amount of time required to vacuum seal a frozen item in a warm air environment could be another factor here, but, if you are not already: I also definitely recommend breaking the seal before trying to thaw them out.
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