You may have done nothing wrong at all. This type of problem happens as a result of weather issues or camellia bud mite infestations. Weather issues is a catch all phrase that indicates that the plant became stressed due to wide swings in temperatures (common in early Fall or late Spring) or by lack of uniform soil moisture (too dry, then too wet, then too dry, etc).

Have temperatures been unusually high and then low recently? Have you noticed any insects on the buds that have fallen (you may have to open them up by removing the layers one at a time).

Here is a link that might be useful: Camellia Info

This is a normal occurance when a camellia is missing one of the trace elements. When the soil ph is too acid or alkaline, the trace are "locked" up . Fertilize this spring with a fertilizer containing "trace" or minor elements. Foliar feed them or adjust the ph to neutral

Frozen camellias and those that have not actually survived root loss due to transplanting may not drop buds and defoliate, showing they are really in fact dead until spring flowering time.