Around 4,000 BC the nomadic and warlike Kurgan people invaded central Europe over a period of several centuries. Perhaps the people displaced moved to England, where they either brought new innovations, or warfare and unrest. After this period of Kurgan invasion, there was almost a thousand years until another Kurgan incursion around 3,000 BC. So if there was no longer a bunch of new people coming in after 3700, then they might have stopped building these centers/fortifications?.
Thanks gleeaikin.