Quite healthy sweet pea buds may suddenly be shed by healthy sweet pea plants. 

The cause is excessive moisture either as a result of prolonged heavy rainfall or over zealous watering during a dry spell.   Little can be done to prevent the former cause but the remedy is simple in the case of the latter and in either event the plant should soon adopt normal growth with bud retention and flower production. 

Sudden changes in temperature may cause the plant to react in the same way and again when conditions stabilize the plants will resume normal growth.