The stock market is set to end May with strong returns, but that may actually work against equities on Friday. A note from the Goldman Sachs trading desk showed that U.S. pension funds are expected to sell $20 billion of equities as part of their month-end rebalancing. That total dollar value ranks in the 86th percentile for net buying or selling in similar rebalances since 2000, according to Goldman. The reason is many pension plans have target allocations for the relative value of their stock holdings versus other assets like bonds or private equity — a large scale version of the traditional 60/40 portfolio. And while stocks have had a banner month, bonds have struggled, meaning that some significant shifts are needed to bring the two groups back in balance when it comes to model portfolios. The SPDR S & P 500 ETF Trust (SPY) is up more than 6% month to date, while the iShares 20+ Year Treasury Bond ETF (TLT) is down nearly 4%. Short-term bonds have held up better, but even Vanguard’s Short-Term Treasury ETF (VGSH) is still on track for a negative month. TLT 1M mountain Long-dated bonds have struggled in May. “We’re not used to sort of seeing the volatility we’ve seen in bonds, as well, and especially when you’re working with something like pension funds or on the institution side, these fund flows can be in the billions easily. And when you start to see those rebalances take shape rather quickly, it can definitely be a short- to intermediate-term needle mover,” said Bret Kenwell, U.S. investment analyst at eToro. To be sure, the selling by pensions could be an opportunity for less rigid investors to buy. HSBC upgraded its view of U.S. stocks to neutral from underweight late Wednesday, in part because of light positioning among long-only investors. Friday could see a chance for some of those to jump back into stocks, especially if they are more confident after apparent progress on tariffs in recent weeks. “Whether it’s the 90-days pause or whether it’s pushing out deadlines with the EU or legal proceedings like we saw last night, I think there’s this sort of belief on Wall Street that we’ve seen the worst of the tariff situation shake out and that we should continue to move toward continued de-escalation,” Kenwell said. — CNBC’s Michael Bloom contributed to this report.