Eye cross belongs to one category of squint, or strabismus in another name, referring to the condition eyes point at different directions. The movement of eyeball is power by six muscles and these muscles are controlled by brain. Hence the incordination might be resulted from imbalance of muscles or disorder of brain. When you have a tired working day, you muscles and your brain are exhausted too. The controlling ability is weakened, in this situation, you might have eye crossed unconsciously. This also explained why some people said they had crossed eyes when they got drunk. In such case, the squint is not very obvious and serious, you can visit your ophthalmologist for help. You can see many children wearing a patching. The patching in covering the good eye, so that the squint eye is forced to be strengthened. After the controlling force of the muscles reinforced, the squint eye is gone, but it is most effective for child before 8 years old. Special designed eyeglasses are prescribed to help patients with squint in common cases, whose squint eyes are caused by vision problem. Some doctor might recommend botulinum toxin injection to weaken the injected muscle and and allow the eyes to realign. While some need a surgery to improve the alignment. As to your case, it is not serious, so I would suggest you to visit your oculist for detailed examination and professional suggestion. Meanwhile, do more exercises to enhence your controlling foce of eyes. Good luck.
Yes, tired can cause eyes cross. Since your eyes are control by your brain and when your are too tired, the nerve of the brain and the muscle can be very weak, so the brain may ignore the image from one of the eyes, and when the brain ignores the image from one eye that eye gets weaker and weaker. That's why you get eyes cross.
It's pretty normal for crossed eyes when you are tired. But you may be the one who have weak eye muscle which make you more easily to get crossed eyes than others. When a muscle in the eye is weak, it can make it difficult to move the eye smoothly.
However, there's no need for you to worry. Take a break when your eyes are tired, use some artificial tears, then you'll be fine.