Wow, am I embarrassed. No wait.. I'm not. What you cited was one of many theories on yawning. What it says is that it happens. Well, duh, we already know that. There are many factors that have been stated as verification that yawning does exist and some of the triggers that may connect with it. What they don't know for certain is WHY it happens and what it achieves. If we are tired, sure. If we are bored, sure again but that wasn't what I said. I said they don't know what causes it. Yawning is a symptom of something and they are not absolutely sure about any specific benefit it serves when it actually happens.
Here's another "random" web search that spells out some of the problems that may connected to yawning. Not why it happens, just that it happened. Sorry to disappoint you.