Ok, there are a couple of things that can go on with an Allison.
Firstly, the mode on mode usually delays the downshift on hills until about 1500 RPM. It prevents the constant shifting when doing the rolling hills. If you're a driver, you already speed up in anticipation of hills anyway so you probably don't notice this. On my Allison, I go to mode when I get to the big roads.
If you have an exhaust brake (PAC brake etc), the Allison ECM will select either 4th or second gear on the pre-select and will downshift to the pre-selected gear when RPM allow it as soon as you start braking. Once you come off the brake and go back to the accelerator, the Allison will shift to the normal gear for that speed and start upshifting again. Some people drive around with the PAC brake on all the time. I don't like the way it jerks out of the lower gear when you step on the gas, therefore I turn on the PAC brake only when I need to slow down.
My guess is you turned on the PAC brake at some point.
