The most common cause of that for me would be weak or improperly positioned connectors. The zirconia likes to be uniform and shrink as a whole. Thin areas tend to shrink faster (for obvious reasons). Connectors...See more that are too far lingual or facial also seem to "bend" the frame slightly due to the thicker, less evenly distributed portions on either side.
CAD/CAM: 3Shape certified, Exocad power user, Zirkonzahn M5/M1, Wieland Select S2s Ask me questions! I love troubleshooting software issues. ;-)