Penny suddenly becoming a bartender on The Big Bang Theory season 4 was due to an unfortunate event involving Kaley Cuoco in real life. Before she became a successful pharmaceutical sales representative, the character was an aspiring actress who moved to California from Nebraska. As she struggled to land acting gigs, Penny worked as a waitress at The Cheesecake Factory, before she unexpectedly started manning the restaurant's bar section in The Big Bang Theory season 4.
Unlike the rest of the Pasadena gang, Penny was the only one who wasn't in the scientific field. That makes her more aligned with the viewers' perspective within the show, as Leonard (Johnny Galecki) and Sheldon (Jim Parsons) typically needed to explain to her what they do for a living. For a time, she started feeling insecure about her intellect, and the fact that Sheldon could be mean about it didn't help. While the rest of The Big Bang Theory cast lacked in social skills, she excelled in mingling. Given this, Penny thrived in being a waitstaff at The Cheesecake Factory, despite some snide comments from Sheldon. So, it was surprising when the establishment suddenly made her a bartender.
The change in Penny's job was motivated by an unfortunate accident. As she recalled in an interview on The Ellen DeGeneres Show in 2010, her horse was spooked during a riding lesson and it caused her to fall off of it. When the horse attempted to leap over her, it inadvertently landed on her leg. Cuoco had to miss two episodes of The Big Bang Theory season 4. At the time, the sitcom didn't make a big deal out of it and didn't even explain why she wasn't around. Eventually, Penny returned to The Big Bang Theory during "The Apology Insufficiency" episode, but to hide the actress' leg cast, the writers decided to put her behind the bar at The Cheesecake Factory as its new bartender. Here's her recollection of what exactly happened to her during her riding accident:
"I heard the crackling [and] I thought he landed on leaves—but that was my bones. I'm not feeling anything, and then I looked. And I was like, 'Wow, my foot is facing me. That's not normal'."
Despite the lack of explanation, it doesn't seem like viewers minded Penny's new role. This was especially true considering that her bartending stint birthed a couple of great bits in the show. One of the best Sheldon and Penny interactions took place while she was manning The Cheesecake Factory's bar. A couple of times throughout this whole duration, Sheldon reached out to her for advice. This was a big step in the pair's friendship as Sheldon usually made fun of Penny and her service at the restaurant, constantly complaining that she wasn't taking his elaborate orders correctly. Yet, Penny being a bartender somehow motivated Sheldon to go visit her to ask for advice for whatever personal problems he was dealing with at that time.
Penny's bartending job lasted even after Cuoco's injury was healed. In fact, it stuck for a couple of years on The Big Bang Theory, so clearly CBS liked the idea even if the last-minute change was due to the actress' real-life accident. Chances are that the writers realized that Penny's new position opened up a lot of opportunities to craft comedic situations more so than if Penny had remained as a waitress.