From the June 6, 2012 commentary:

When I wrote this scene, many months ago, I was not specific about what Luis dropped. It varied between being a utensil, a jar of peanut butter, and other items. But it ended up being a pan he knocked over. Not that the object was important, of course. I just wanted him to find himself in a situation where his condition is an obvious — and frustrating — handicap, to both sort-of justify his crankiness and open him up to a meaningful discussion with his brother.

Not that I’ve ever been laid up in a wheelchair for an extended period, but I did have an appendectomy a few months ago, and it left me feeling as physically helpless/useless as I ever have in my life. I’m a very active and agile person, and have been since I was a kid. So for me to be unable to even do basic things like sit up or use the bathroom normally was immensely frustrating. So, in retrospect, I definitely was projecting some of my own expected frustrations onto Luis, both in this particular strip, and in his characterization in general. He’s a kid who was a promising basketball player whose ability to play the game (in his eyes) was stolen from him. That has to make this new reality for him doubly frustrating.