Glass jars can break in the freezer if you seal them before the contents are completely frozen. Liquid expands quite a bit as it is freezing, and if you have a lid tightened on the jar while freezing so much pressure will build up that the glass can shatter!
I fill the jars up to within 1 inch (1 1/2 inches for larger jars) of the top to give the liquid room to expand, and I leave the lid off while freezing. Once the contents are completely solid (I usually wait till the next day) I screw the lids on all the mason jars, stack them up in the freezer, and that is it till I need to use them!