Why is it that when i am doing walk aerobics at a fast pace that i start yawning even when i am not tired?

Lung expansion: This is the way your body fully expands your lungs.

Need for oxygen.: A yawn implies that your body needs more oxygen at that moment, and your brain triggers a gigantic inhalation, i.e. a yawn. During your exercise, you likely need more oxygen at that moment than your body can deliver, so you yawn. Not typical, but not unexpected.