Solder joint cracking is normally found after some form of environmental testing, hopefully not found at all. In manufacture it can only be caused by flexture of the board after soldering. Areas which should be checked for the possible causes of flexture are in circuit test, break out of multi panels, tight reflow or wave solder fixtures and, of course, just poor handling.

An interesting often forgotten, cause of open joints is wave solder topside board temperature. Not exactly a cracked joint but the visual appearance is the same. When the board goes through the wave topside joints reflow. As with even the best systems boards flex leaving some joints open. It can also happen during second side reflow if only some joints become molten.