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The NPR News article and podcast Scientists Share Results From NASA's Twins Study says:

SCOTT KELLY (NASA Astronaut): You know, the symptomatic stuff is fine. I don't have any long-term negative feelings, physically, from being in space. Now, there's the things you can't feel. And hopefully, I will never learn that those are a problem.

GREENE (Host): Those things you can't feel - well, it turns out they are as small as the protective structures at the ends of his chromosomes.

MARTIN (Host): Yeah. These are called telomeres, and normally, they get shorter with age. But what about in space?

SUSAN BAILEY (Principle Investigator): What we wanted to do was evaluate telomere length in both of the twins before and after so that we could see, you know, where they started and then where they ended.

MARTIN: Susan Bailey was one of the scientists who answered this question. She expected the stresses of space to shorten telomeres quicker.

BAILEY: And, in fact, we saw exactly the opposite thing - that during spaceflight, he had many more long telomeres than he did before he went up. So that really couldn't have been more of a surprise to us.

See also Radiation Biologist Dr. Susan Bailey Studies the Cellular Clocks of Astronaut Twins

Question: Why did investigators initially believe that Scott Kelley's year in space would accelerate the rate of telomere loss, relative to his baseline rate and the rate of his identical twin brother on the ground? What would be the postulated mechanisms?

Dr. Bailey wrote a short piece that hints at the factors behind her initial expectations:

Telomeres are the ends of chromosomes that protect them from damage and from “fraying” – much like the end of a shoestring. Telomeres are critical for maintaining chromosome and genome stability. However, telomeres naturally shorten as our cells divide, and so also as we age. The rate at which telomeres shorten over time is influenced by many factors, including oxidative stress and inflammation, nutrition, physical activity, psychological stresses and environmental exposures like air pollution, UV rays and ionizing radiation. Thus, telomere length reflects an individual’s genetics, experiences and exposures, and so are informative indicators of general health and aging...

Our study proposed that the unique stresses and out-of-this-world exposures the astronauts experience during spaceflight – things like isolation, microgravity, high carbon dioxide levels and galactic cosmic rays – would accelerate telomere shortening and aging. To test this, we evaluated telomere length in blood samples received from both twins before, during and after the one year mission.