>>11570409I've had it happen with no known jostle of the shelf. The weight can gradually exert pressures on the unratcheted joints like the torso joints and toe hinges to shift weight--especially those toe hinges which if they bend forward even a little bit can cause a dive with no puck.
The toe joints are not always stable on all mcfarlanes obviously. There's a range based on QC chance and your specific figure. Some come pretty loose, some come super tight, some in the middle. Some toe hinges on some of my mcfarlanes barely move unless I force them, and others are floppy and move on their own, some seem in the middle.
And the torso moving a little bit can also cause a dive since it causes a weight shift. More likely to happen the heavier the upper body is. Very unlikely to happen on normal female figures but some stuff like figures with heavy wings can be top heavy. Obviously balancing them backwards can help so if they fall they go backwards but their legs can kick out figures in front of them if they fall backwards as well so still could lead to another figure in front of them taking a dive. Generally I try to secure top heavy figures much better than normal ones even with the puck. A few like the early Armored Superman are so top heavy they can actually lift the puck itself if posed in certain ways.