MDMA is a stimulant, and granted the next statement is in regard to exercising, maybe it could apply to the stimulation of the body as well:
"Yawning is often caused by taking repeated shallow breaths when working out. You may not be breathing deeply enough to supply your lungs and bloodstream with adequate oxygen for strenuous physical activity."
Or simply:
"Excessive yawning may also be the side effect of a medication you are currently taking, including antidepressants and medications for Parkinson's Disease."