What would be the cause of unexpected acceleration while braking a KIA Rondo LX?
I never owned or drove a Kia, but the first thing I would look for is mechanical interference between the brake pedal and accelerator, either inside the cockpit or under the hood.
I doubt that there would be a “the” cause. Instead, I suspect that there are multiple possible causes, ranging from you had your foot on the gas pedal to a defective vacuum booster.
What year and engine? Have you asked the dealer if there are any TSB’s about this?