It is normal. You have what is called 'naso-lacrimal ducts' these are tear ducts that are in relation to your nose- like when you cry, your nose runs! So any respiratory activity- like a good yawn- can bring on the tears! Usually a big, long, deep yawn would stimulate the tear ducts.
This is often an automatic response that your body does and does not always happen when yawning. Fi it does, it is the body telling you how overtired it is