We May Finally Know What Causes Kevin & Randall’s Rift On 'This Is Us'

When the This Is Us Season 4 fall finale ended, fans were left open-mouthed by the twists thrown at them. But nothing was more shocking than the revelation for the Season 5 premiere, which indicated Kevin and Randall were going to fall out so badly that they wouldn't spend their next birthday together. Since then, fans have been watching for any sign of what causes this breakup. Now, a new theory suggests things are more complicated than a simple argument. Fans are starting to ask: Is Randall and Kevin's fight about Rebecca and her condition?

Spoiler alert: Spoilers for Season 4, Episode 15 follow. Viewers have good reason to believe Kevin and Randall will reach an impasse over this subject. Rebecca's diagnosis has come back after the MRI, and all signs point to the onset of Alzheimer's disease. Rebecca's specialist ended the episode with recommendations for a specific course of action for the medical side, but how Rebecca chooses to live the days and months ahead, knowing her mind will slip away sooner rather than later, is an open question.

Rebecca may have the disease, but in his first therapy session, Randall viewed it as his problem to solve. When Jack died, Randall took on the responsibility of "being there" for Rebecca every weekend. He helped her get a job, taught her how to use computers. It was clear from what Randall told his therapist that he genuinely believes the family got through that tragedy of losing Jack because of him, and him alone.

He also revealed he still sees his siblings the way they all were at 19, with Kate soft and helpless, and Kevin irresponsible. When he found out Kevin would be the one taking Rebecca to get her test results, he scoffed in frustration. As far as he's concerned, it's a disaster; Kevin will fail to keep track of time, and mom won't make her appointment.

But out in California, things weren't what Randall imagined them to be. Rebecca had taken to a "carpe diem" philosophy: Forget the responsibilities, forget the drudgery, seize the day, do it all, now. She was the one trying to convince Kevin that they can skip her appointment and do something fun instead. It was Kevin keeping tabs on time, gently pushing his mother back on track. Sure, he would indulge in her whims, but he also wouldn't let her miss the essential things.

Randall's inability to recognize his brother is no longer a flake is a huge red flag. On the one hand, it's proof Randall isn't nearly as in touch with his family dynamic as he thinks. But more importantly, it suggests part of him can't let go of his image as "the responsible one." Randall needs Kevin to be a screwup, so he can still be mom's hero.

If this theory holds, then Randall is, in a sense, stuck in the past, with his anxiety coping mechanisms keeping him viewing his family in static terms. Therapy will hopefully help Randall see Kevin and Kate as equals as the sessions go on. But how long will that take? How much of his mother's final years will he miss before he can accept Rebecca's problems are too big for him to handle alone?

This Is Us continues on Tuesdays at 9 p.m. ET on ABC.