4 Answers

Permanent teeth come in crooked for different reasons. That's when a person would go to an orthodontist to have them straightened. Usually that's about the time all your baby teeth are gone and the permanent teeth are coming in. In that case, you can say you were born that way.

If you have teeth taken out later in life, sometimes that can make your other teeth move and become crooked.

Your teeth change as you grow, developing and growing like all other bones in your body. There is a genetic disposition, but crooked teeth, etc. are not caused at birth. Even if you had corrective dental treatment, like braces, as a kid - in later life the teeth can become crooked again and require more work. 

It happens when your adult teeth start coming in crooked, or to high up in the gum, or too many teeth for your size of  jaws. . . . . . some are overlapping and have to be pulled out before  you get braces.  If you're an older adult, and you get teeth pulled, and don't get them replaced, then your teeth can go crooked, or get too spaced apart;  That's what happened to my brother, and my mother was really appalled about that. I don't know what she was doin lookin in his mouth

country bumpkin

Just be glad she wasn't looking in some other cavity? LOL (I couldnt resist the play on words)

Malocclusion is largely genetically determined, but jaws are getting smaller due to under-use as are all the bones of the body. That is why impacted wisdom teeth are becoming more common. Gum disease can also cause tooth movement.