Hi
the Crumbling event relates to the Cluster Crumbling features
In blueprint for example you can ask a cluster / geometry collection to fully crumble ( CrumbleCluster or CrumbleActiveClusters functions on the GeometryCollectionComponent )
What this does is breaking all the children of the cluster from their parent
Crumbling can also happen if you use the Remove On break feature
This feature allows you to decide how fast a piece disappear/shrink after it get “broken off”
( You can configure this in the Utility section , Remove On break tool of the Fracture editor tool )
If you want to know if something broke in an event, you can simply use a break event
it will be generated per piece broken
Hope this helps