Q: Auto transmission downshifting to lower gear when decelerating causes the car to lurch. Why?

This may be a result of low transmission fluid or potentially a faulty transmission control module. A faulty transmission control module may cause a delay in acceleration or erratic shifting of gears. The transmission control module has various sensors such as vehicle speed sensors, wheel speed sensors, throttle position sensors, turbine speed sensor, and transmission fluid temperature sensors that all direct information to it in order for it to perform it’s function. Other components controlled by the TCM are components such as a kick down switch, traction control system, and cruise control module. Any of these may also affect the way the transmission control module performs and controls the transmission and the shifting of your vehicle. In your case it sounds like you may have a faulty kick down switch causing the transmission to downshift prematurely. I would suggest having a professional from Your Mechanic come to your location to properly diagnose your transmission shifting problem.