As the zone thermostats reach their setpoint and the associated zone valves close, the distribution system as a whole develops increasing resistance to flow. The overall system flow rate decreases, but the flow rate through each open zone circuit increases. Some of the lanes on the bridge are now blocked off, and it's simply not possible to get the same amount of traffic across, even though traffic moves slightly faster along each lane.
The system curve of the distribution system steepens each time a zone valve closes. Figure 2 shows this effect for a system having several identical floor heating circuits each controlled by its own zone valve.