Well, the obvious reason is that they are tired...but our trainer told us yawns can have other meanings. She said it can also be a form of body language, that dogs may yawn when they are anxious, stressed, or looking for reassurance. I had never noticed this before, but she told us to pay attention for a week and get back with her at class. Sure enough, Shelby would yawn when we didn't take her out right away when she whined to go outside, etc. She'd be anxious to go, sit down by the door, and yawn periodically until we let her out. She also yawns when she senses the dog park is only a few turns away...she gets excited! Another time we noticed is when we would practice a new trick with her and she didn't know at first what we wanted. She'd yawn as if to say, "am I doing what you want?" Anyway, those are just a few examples. Try picking up patterns the next week and you'll be amazed!