My understanding of it is that the ice crystals will break cell walls and fibers due to expansion. Once you thaw it, more liquids will be released due to the cell damage.
Effect of freezing on sensory quality, shear force and water loss in beef
M. longissimus dorsi
Å. Lagerstedt
, a, 
, L. Enfälta, L. Johanssona and K. Lundströma
aDepartment of Food Science, Swedish University of Agricultural Sciences, P.O. Box 7051, Uppsala SE-750 07, Sweden
Abstract
The objective of this study was to determine how sensory quality, shear force and water loss differ between beef stored either chilled or frozen before cooking. Meat tenderness was analysed instrumentally and sensorially using both a consumer panel and a semi-trained panel. Both
M. longissimus dorsi (LD) from eight young Holstein bulls were cut into eight samples, weighed, vacuum packed and aged at 4 °C for 2, 7 or 14 days. After ageing, the frozen samples were kept at −20 °C prior to heat treatment. Water holding capacity was recorded as purge or thawing loss and cooking loss or as combined loss. Sensory analyses were performed on samples aged 7 days. Peak force values declined with ageing time and freezing. Frozen meat aged 2 days had the same peak force values as chilled meat aged 7 days. Total energy was the same for both treatments at day 2 and 7, whereas at day 14 frozen samples showed significantly higher values than chilled samples. The sensory panel experienced the chilled meat to be more tender, juicier and having a more intense meat taste than the frozen meat, whereas the consumers could not find any significant difference in degree of liking. Water holding capacity was lower for the frozen samples. The results indicate that conclusions from studies concerning sensory quality of beef will depend on whether the meat has been kept chilled or frozen before testing.
Here's another:
http://www.extension.umn.edu/distribution/nutrition/DJ0856.html
Freezing
Freezing rate plays a small role in tenderness. When meat is frozen very quickly, small ice crystals form; when meat is frozen slowly, large ice crystals are formed. While the formation of large crystals may serve to disrupt components of the muscle fibers in meat and thereby increase tenderness very slightly, the large ice crystals result in an increased loss of juices upon thawing. This increase in loss of juices results in meat that is less juicy upon cooking and therefore usually is perceived as being less tender.
