because

when you’re in the same environment as them, you’re exposed to the same temperature

your brain is slowing down, causing its temperature to drop

your brain isn’t feeling stimulated and starts to slow down, causing a temperature drop

Even thinking about yawning can cause you to do it. It's something everybody does, including animals, and you shouldn't try to stifle it because when you yawn, ...You yawn when you’re:Seeing someone else yawn:Tired:Bored: