Too much success can sometimes draw the ire of colleagues, as demonstrated by Morgan Stanley’s announcement on Wednesday that it will no longer offer Vanguard mutual funds to clients starting next week.

According to the brokerage firm, which has close to 16,000 brokers and $2.2 trillion in assets under management, the move is part of a broader overhaul of its mutual fund offerings. Since April, Morgan Stanley has been reducing its mutual fund menu by about 25% to eliminate those it deems to be unpopular, underperforming or a conflict of interest. The purge is part of the firm’s commitment to complying with the Department of Labor’s fiduciary rule on retirement advice.

A spokesman of Morgan Stanley told The Wall Street Journal that Vanguard mutual funds are not popular with its clients, and represent less than 5% of its total mutual fund assets.

Industry insiders however believe Morgan Stanley is retaliating against Vanguard for its longstanding policy of not paying brokerage firms to sell its funds, otherwise known as “shelf space,” which is part of the reason why the asset manager can offer record-low expense ratios to investors. Most of Vanguard’s competitors pay Morgan Stanley $250,000 to $850,000 a year, reported AdvisorHub.

Morgan Stanley is not the first to adopt this strategy. A year ago, Merrill Lynch began restricting new sales of Vanguard mutual funds to clients who do not have an existing position.

A similar policy applies to Morgan Stanley accounts, the WSJ article shows. Clients who are currently invested in Vanguard mutual funds will not be forced to sell, but only have until the first quarter-end of 2018 to add to their positions. Both Morgan Stanley and Merrill Lynch continue to offer Vanguard exchange-traded funds.

Since launching the first index mutual fund in 1976, Vanguard has become the industry leader in index investing and low-cost funds. It is currently the second-largest asset manager in the world, after BlackRock, and boasts $4 trillion in AUM. An estimated $1 billion a day has flown into Vanguard funds since the 2016 U.S. election, more than 8.5 times the rest of the mutual fund industry, according to Morningstar data.

Vanguard’s near-monopoly position is part of the reason behind Morgan Stanley and Merrill Lynch’s efforts to stymie some of the firm’s incredible growth and protect their own revenue models. The brokerage arms of Wells Fargo and UBS, the other two of the “big four” brokerage firms, or so-called wirehouses , have not followed suit and continue to offer Vanguard mutual funds.

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