The word 'yawn' is both a verb (yawn, yawns, yawning, yawned) and a noun (yawn, yawns). Examples: Verb: I began to yawn in the middle of the lecture. Noun: The comedian's joke met with a big yawn. An adjective is a word that describes a noun, for example: a big yawn.
People yawn when their brain isn't getting enough oxygen. If you not breathing properly and not getting enough air while singing, you will yawn. If this is an annoyance to you, practice proper breathing techniques for singers. You can find them anywhere online.