Tightened the rear pads? Did you mean adjusted the rear brake shoes? The rear wheel cylinders might be faulty.

 

My 2011 did this. Well, not exactly the same. Didn't go quite to the floor. But it would operate as normal under light braking, but snap down to a much lower level with just moderate force. It's wife's daily driver. She prolly never noticed because she actually drives carefully.

She also backs up gingerly. I thought about that, suspected self-adjusters were going unused, backed up briskly several time and popped the peddle a total of about 40 times. Seems to have solved the problem. I was told in these forums that I was luckly. Wife says I lead a charmed life. She exaggerates.

After a new brake job, the brakes settle in and need a good bit of self adjustment. Conclude what you like.