I've thought that this could be an ongoing issue with Caleb this season and before. Often when you injure the lower back (depending on what the injury is) even when the initial damage heals, scar tissue can make you less flexible, and can cause additional stress on the discs and vertebrae.. Anything that causes irritation or stress, (especially if prolonged or repeated) to the previously injured area can possibly result in recurrent inflammation, spasms, stiffness, and pain. Can throw off your balance as well. I'm pretty sure I noticed a back support under his jersey previously this season.
When you have a bad back and especially if you're an athlete at top levels of competition, it's often an ongoing process to keep a chronically and intermittently bad back from affecting performance, including routine back stretching and various other treatments, avoiding unnecessary stress to the area, having enough time for game/practice and travel induced lower back inflammation to subside, etc... If that's what Caleb has been going through, then I certainly can understand any possible lapses in performance he may have had on occasion, and I can definitely empathize with him.