Steam Acses Braking Out Of Nowhere

Can someone explain why, on the MBTA Commuter, the speed limit just drops out of nowhere without any possible way to be able to brake in time so the ACSES applies a penalty brake.

I thought that when there was a yellow signal, you had to brake below 45mph but the limit already drops way before the yellow signal on just a straight peace of track without any indications. Then, at the yellow signal it drops to 30 and then like a mile later it drops to 20, again on a straight peace of track without any indications.

I wanted to ask how it's possible to avoid the brake applications if there isn't any way of seeing when the speed will change. (I also had instances where all signals ahead were clear, but the speed limit just drops to 30 and applies the brakes for no apparent reason.)