Why do my ears open up when I yawn big? I have tmj

Because of this... Your ears open up because when you yawn you open up the eustachian tube that connects your throat with your ears. It probably has nothing to do with "tmj", unless the jaw joint itself is popping, which would indicate another problem altogether. In other words, you might have a displaced disc in your tmj's if that's the case. Otherwise, it's normal for your ears to open up when you yawn widely.
Opening tubes in ear. Everyone has two tmjs. What you are alluding to is temporomandibular joint dysfunction, when the jaw doesn't function normally. The opening sound you hear is when the tubes in the throat which connect to the inner ear open and the pressure is equalized on both sides of the ear drum. This may or may not be associated with your tmd.
Eustachian tube open. Your eustachian tube opens when you yawn.
This is normal! In the back of our throat, there are muscles that are very close to the ear canals. Every time we yawn, these muscles distend and "pull on" another small muscle attached to the ear canal, causing pressure in the ear to normalize. As a result, our ears "pop". You'll find that if you need to pop your ears on a plane, you can do so by swallowing hard!