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This occurs mostly in the epiphytic cacti. It is caused by lack of water, dry air or too high a nitrogen feed.
Also moving a plant can cause a cold shock, as can spraying with very cold water.
Once buds are quite well formed, if plants need to be moved, they should face the sun the same way as they did in their previous position, otherwise buds will twist themselves off, trying to follow the light.
Buds on desert cacti can drop if they are given too much water, too early in spring eg. Echinopsis, Rebutia, Lobivia and Opuntia.