Usually tail kinks result in unstable temperatures during incubation or due to genetics. Most often it is unstable temps but it isn't uncommon for it to occur in a gecko who's parents had a kinked tail. You would need to test breed it to be sure.
Enigmas sometimes have a very slight kink to the end of their tail. As stated above, it is usually either a temperature fluctuation or a genetic problem. Test breeding and tail kinks can be a very sensitive subject. That's why most folks just plain don't breed anything that has a tail kink or curl.
Which reminds me, I forget if it was this forum or another. There was a person who had a leo with a curly tail, ended up breeding it, produced a lot more curly-tailed geckos, and then decided it was cute and posted pictures of everything. Needless to say, there wasn't much positive response. I wish I had the energy to search for the thread.
I lost power to a storm last summer. We were out power for a good 10 hours or so. I had eggs incubating around 25 days and notice temps dropped to 77* from 83ish and one of the hatchlings from that time had a tail kink. I'm not 100% sure if that was the cause but it makes sense.
Adding to what has already been said:
Stress can bring on slight tail kinks as will a lack of vitamin A I believe. I have had geckos shipped to me and when I received them they had slight tail kinks. After a few weeks in my care the kink went away.
Giving your geckos too much calcium can actually block the absorption of vitamin A leading to a deficiency. Which is why I provide a well balanced vitamin supplement, containing vitamin A, calcium, and D; rather than just straight calcium/D.