I have brass 1/2" male x 3/4" female threaded fittings in a chilled water system. After 6 months to a year in service the 3/4" sides have started to crack and leak. Can't figure out why.

Over tightening the brass fitting could cause it to develop a crack in time.
Or, I have had brass angle stops develop cracks because the brass itself was faulty coming out of the foundry. Take your pick. Cheers, Tom