Key paragraph (from the original FT article):
The issue has arisen from idle cash sitting in customer accounts at brokerage firms and large banks, which “sweep” otherwise uninvested funds into interest-bearing alternatives in order to generate income. The SEC is looking into whether the firms steered those clients into sweep accounts that paid little or no interest, and whether the financial advisers at those groups had a fiduciary duty to advise clients they could make higher returns if they moved their cash into other accounts.