Why did they expect Astronaut Scott Kelley's telomere shortening to accelerate? (they got longer!)
Dr. Bailey wrote a short piece that hints at the reasons behind why she expected what she expected:

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.