My first thought too was pedal placement. I've done this before.
The brake and accelerator are intentionally placed pretty close to allos a driver to press the brake and "blip" the throttle (tap the raise the RPM's to match speed) on downshifts. On an X3, there is no real reason for this anymore since the computer blips the throttle for you on downshifts (my experience on 2013 X3 28i) and no manual available here in the US. I'm sure they keep the pedal placement consistent across their entire fleet of cars where it is practical to do this. Make sure to rule this out when you get a chance by either lifting off brake and trying again and making sure you're not nudging the throttle.
I don't think it's a malfunction of the accelerator pedal as there are redundant checks and balances that analyzes it constantly for any possible errors and immediately goes into "limp home" mode if an issue is detected.
2013 X3 28i (prem, tech, cold1&2, conv, hud)