Investors poured $26.4 billion into taxable-bond funds and another $5.6 billion into municipal-bond funds last month, according to data from Morningstar.

However, U.S.-stock mutual funds and ETFs bled $22.4 billion in August, making it the worst month in two years and the fifth worst during the past five years for the asset class, noted Kevin McDevitt, Editorial Director at Morningstar. International-stock funds had $2.8 billion in outflows, the group's worst showing since December 2011.

On the fund level, Old Westbury's Global Opportunities fund accounted for the bulk of it $1.4 billion in inflows while American Funds logged another $5.5 billion in outflows, according to McDevitt.