If a large number of half-folded multi-sheet is delivered, the edge of the sheets may bulge and some part of the edge will be swollen. If this happens, other sheets loaded on the bulged paper may turn over in the output tray.
This may occur if:
Thick, relatively stiff paper is used.
When 127: [Process Speed Setting: Productivity] or 128: [Process Speed Setting: Quality] in [Advanced Settings] for the custom paper you are using is set to [Low].
As a bundle is delivered, its folded edge may droop and catch on the stacked bundles, causing the delivered bundle to flip over.