Tunnel vision is caused by loss of peripheral vision. Usually it is normal to get tunnel vision if you stand up quickly. I don't know how to explain it, but most of people suffer it. It is not a serious problem and the tunnel vision will recover soon about several seconds. Also, i heard that blood loss to parts of the bran can also cause tunnel vision. However, if you often suffered from tunnel vision, you shall pay attention to it. It is said that tunnel vision can also be caused by retinitis pigmentosa and glaucoma. So, you'd better check your eyes if you suffered from it.
Actually, this symptom has something to do with your blood pressure. When you are in this situation, you need to be very careful in case you may fall down. Honestly, most of people have experienced this. Maybe you need to have an overall exam, which can also make yourself rest assured. Next time you had better stand up slowly and try to control your mood in your daily life. Some people also said that it may have something to do with genetics. Anyway, I wish this could help you.