Rodents are often flash frozen(after being euthanized, naturally) and this freezes them quite hard and very fast. When packing them, parts often get broken because the body is not brittle. A gash could be from the sharp nail of another rat when it was pushed into the freezer bag(while frozen).
If the damage looks fresh(not a healed over stub) then it was probably done after freezing. Some places will kill the rodents, then pack them in a tray or bag, then freeze the whole thing. This has the advantage of a neatly compact package, but sometimes it's harder to separate without thawing the entire bag too. The heat from a pile of warm bodies also takes longer to disperse, which could mean they are not quite as fresh when thawed.
I hope this helps!
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