Greetings Ives,
To my knowledge the hematoma can be attributed mainly to two reasons.
The first is in the breeding process as chickens which are sold as broiler/fryers typically are slaughtered between 6 to 8 weeks of age. Outwardly they appear fully mature, but their bones haven`t completely calcified. As a result, the bone mass is very porous. Deeply colored pigment from the bone marrow migrates to the surface and often is visible along the bone and the meat that is immediately attached to the bone. The ice "dilutes" it a bit and the thawing process may push this redish pigment further up on the meat.
Second reason could be animal mistreating and/or high temperatures while it is alive, just prior to slaughtering. Mistreating (like kicking/hitting with hands or sticks or extreme crowding of the birds in the waiting hen) can cause bruises which will not have time to fully develop and start healing, because of the death of the animal. So these bruises due to ice and subsequent thawing will have the appearance of hematoma. This can sometimes be evident if you cut the spot and find underneath a thin layer of red jelatinous hematoma. In this case you should do also an unannounced supplier audit and evaluation in search of the problem, as this falls into animal wellfare and you don't want to be a part of something like that, even going as far as employing the services of a veterinary for this audit (my suggestion).
There could be other explanations that elude me and experts here can give more information.
Regards!
