There are many reasons that caused the filter housings to be cracked:
The most common reasons are freezing or hitting the housing on the floor during filters changes or during assembly set up by homeowners and causing the housing a fine "hair cut" crack that will be not visible to their eyes vision and might develop after days/weeks months which depend on the cut size, weather and water pressure and will end up to major crack any time afterward.
The environment the system was set up in is too cold causing the water circulating through the system to freeze and crack the housing.
The pressure in the system is too much and over 90 PSI, causing cracks. This is due to customer error as Max Water provides in the system description the threshold of pressure the system can withstand.
The Housings were twisted open/closed without using a housing wrench, or other forms of user mishandling.