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 is likely to occur if:
Thick, relatively stiff paper is used.
As a bundle is delivered, its folded edge may droop and catch on the stacked bundles, causing the delivered bundle to flip over.