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When sintering try leaving the sprue and base on and raise the tempture in 5 degrees per minuete but it varies based on furnaces.
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 that are too far lingual or facial also seem to "bend" the frame slightly due to the thicker,...
Try to leave a piece of the milling blank attached to the bridge during centering. Some nesting software have runner bars you can add to the bridge for centering.