Style drift happens when an active manager drifts from a specific style, asset class, or index that is described as the investment purpose of a portfolio or mutual fund. For example, a manager may drift from small cap value to small cap growth. This is a substantial problem if you have carefully determined your Risk Capacity and matched it to a Risk Exposure.
Hypothetically, you may be intentionally invested in a growth fund. Then unbeknownst to you, your active manager takes 30% of your Large Cap Stock fund and puts it in cash and bonds. This changes your growth fund to a balanced fund, changing the risk exposure, return, and time horizon of your investment.
To avoid style drift, it is best to implement your asset allocation with "pure style" index funds. Index funds are invested using clearly defined rules of ownership. Forty percent of the time, actively managed funds follow a manager's drift to a market that the manager thinks will keep his shareholders happy and save his own hide. Unfortunately, the shareholders suffer in the long run. As we have seen in previous steps, this predicting or chasing of returns has resulted in "below market" performance.