Phal. spikes in particular don't like cold, dry, or changes in conditions. If the plant is stressed or has rotted/dead roots the stress will cause bud blast. Lack of humidity or plant too dry, likewise. Also, if the plant is moved from one set of conditions to another the buds can blast. A possible, but less common reason is ethylene gas-a source can be ripening fruit like bananas. It can also happen occaisionally for no obvious reason, however, if it is happening regularly there is a problem.
I don't grow Dendrobiums, so can't speak to those specifically. Some of the reasons above could potentially apply.