Conclusion
I learned that nobody really knows how tornadoes are formed, because nobody has been able to get that close to one and survive.  My hypothesis is based on what experts believe, but your guess is as good as mine.

We know thunderstorms develop in warm, moist air. Rising air within the thunderstorm updraft tilts rotating air from horizontal to vertical.  An area of rotation extends through the storm. Intense tornadoes form within this area.

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