Using a regression model the authors set out to examine the tendency for fund returns to exhibit positive correlation over time.  They conclude that company size and fund manager investment styles are the most significant factors contributing to mutual fund momentum, while market risk and fund expense ratios have little effect.  Here is their reasoning:

What Drives the Momentum in Mutual Fund Returns?
Robert A. Weigand, Ph.D., University of Colorado and
F. Larry Detzel, Ph.D., California State University