One of Sheldon Cooper's most lovable quirks is his signature three knocks before opening any door. In Season 10 Episode 5 of "The Big Bang Theory," he finally reveals why he does this and the reasoning is actually quite heartbreaking. In "The Hot Tub Contamination," Sheldon discloses to his frequent confidant Penny (Kaley Cuoco) that when he was 13, he came home from college for spring break to an extremely unexpected and unwelcome homecoming.
To preface, Sheldon was 12 at the end of Season 5, possibly setting us up for this climactic moment in the spring. He hears odd noises coming from his parents' bedroom and inquisitively investigates to find his father in bed with another woman. Most likely, Sheldon caught his father with Brenda, who George was almost involved with in previous seasons. The traumatic moment forever shaped young Sheldon, causing him to adopt this formula that gives one ample time to "get their pants on," as grown Sheldon indicates.
In Season 6 Episode 2, George and Brenda have an awkward and heated exchange in the chicken coop shed. They engage in a brief spat over their almost-affair and George warns having Mary so close at work is a bad idea. After Mary and Brenda drink together, thus forming a closer bond, George may be right in worrying. Then again, Mary seems to have another man on her mind, which could also drive George into Brenda's open arms.
