The source website mentions bird migration. Without doing any research, seems like the radar picks up any objects in the atmosphere, whether rain, birds, or I'm going to guess even insects etc. The radars are operating continuously, so the "pulses" have nothing to do with radar being turned on and off or power being changed. It's reading something in the atmosphere.

The way the increased activity moves across the US from east to west seems to track sunrise and sunset times. And it's increasing into the spring. My best uneducated guess would be it's picking up increased bird or insect activity that's on a seasonal increase and also very correlated with time of day.

Getting more specific would take some research, but that's the angle I would start with.