Lower oven won't turn off

I have a 24" KitchenAid Superba double wall oven with pushbutton electronic controls. Yesterday I had both ovens cooking and turned them both off. The display went black normally, but the lower oven unit didn’t turn off. I woke up at 3:00 a.m. and heard the oven fan going. The lower unit was still on. It was still hot. I pressed the off button several times–nothing happened. I finally had to trip the breaker switch to turn the oven off. This morning after the oven had cooled off, I turned the breaker switch back on. I did not touch the controls to turn either over on or off, but the lower oven started heating up again. What’s the problem?