BFP#3 12/09/12 - EDD 8/11/2013 - D&C @ 12w :: BFP #4 2/17/14 - EDD 10/28/2014 - M/C @ 6w
It's because BM shouldn't have a drastic change in temp. - or at least that's what the LC at my mommy's group told us. Yes, fresh expressed milk is considered warm.
FYI: She also told us it shouldn't go from boob to freezer, to put it in the fridge first - gradual changes are better... also freezer milk should be thawed in the fridge.