"Work at Silbury Hill also ceased.   For decades, generations, possibly a century or more, the project had continued, drum built upon drum, the network of cells packed with chalk, the stepped sides filled and smoothed into elegant slopes.   Then it stopped.  The topmost drum was left unfinished, jutting sharply upwards over the dome below ..... it was unlikely to have been boredom or weariness that led to the break.   The probable cause was danger.   Some time between 2700 and 2600 BC work on the third stone circle stopped.   By 2400 BC men were no longer toiling on Silbury Hill.   Instead, around 2500 BC, work gangs began the digging out of a deep ditch and the raising of a fortified bank around Avebury's stones and settlement".   ~ Burl, p 185-6

Terence Meaden, however, has a different emphasis as set out  in his book Goddess Of The Stones. If I understand him correctly, he basically posits that the spiral patterns carved naturally into grass and crops by whirlwinds were interpreted in neolithic times as sacred manifestations of the Great Goddess or ancient spirits, and were used by our predecessors to created outlines of many small stone circles and barrows.   He suggests further that a mound of turf at the very base of Silbury might have been such a spiral record of divine presence, and venerated accordingly with the construction of a massive marker, the Hill itself:

“The internal structure comprised several sections …. Only in the innermost mound were materials other than chalk found. Chief among these was a circular stack of turf …. hallowed in the eyes of the people who took such pains to select and consecrate it …. in a world dominated by magic and religion the turf was infused with their spiritual adoration, and this profoundly holy respect elevated the dark inner core to the status of priceless treasure” (pp 163-4)

“Because of the precision of the builders, it was possible to hold ceremonies on the flat summit in the knowledge that they were taking place exactly above the sacred spiral turf which the participants knew to have been blessed and consecrated by the ascension or resurrection of ancient spirits … . sorcerer or sorceress, shaman or divine arbiter, priest or priestess – whoever it was, he or she took centre stage upon this spiral mountain in the light of the rising moon or the rising sun” (p175)