The veterinary term for the condition is 'synchronous diaphragmatic flutter' (aka thumps). It occurs as a result of phrenic nerve being stimulated as it crosses the heart. The phrenic nerve seems particularly sensitive to changes in electrolyte concentrations. The condition is quite commonly associated with low calcium levels. In endurance horses it is electrolyte loss that causes the problem in non exercising horses it is less clear. However treatment with calcium almost always will make the problem disappear. Beware that the inciting cause such as an illness or something else may still be present.