Why our tounge start jiggling​ when we open our mouth infront of the mirror?

If we talk in short this is due to the nerve impulses in the tounge.The tounge has distribution from three different sensory nerves branches.

The muscles of the tounge get distributed by one nerve except one muscle which is joined by the soft palate to the tounge. Hence jiggle overdue to differences in the direction of the impulses of two different nerves at different time. If both go together it will not possible for the tounge to take the food inward.when keeping the mouth open this jiggling happens automatically