Greenwood and Thesmar (2011) argue that
active mutual fund owners of stocks can have correlated trading needs and thus the stocks that they hold can comove, even if there are no overlapping holdings.
Greenwood and Thesmar show that these correlated trading needs predict future price volatility and cross-sectional variation in comovement.
Greenwood and Thesmar acknowledge that a concern with their interpretation is that funds with orrelated flows might simply choose to invest in similar stocks whose fundamentals comove.