If I freeze a sauce, particularly something with mince in it, when defrosted it produces excess water.
The sauce before frozen is nicely thickened.
I think it might be because the proteins in the meat are damaged in some way by the ice crystals so that they are unable to retain as much water.
EDIT The sauce I'm referring to is bolonaise and the only thickening agent I used is a bit of tomatoe puree.