U.S. stocks narrowly preserved their August gains after a volatile month, but market watchers say conditions have become increasingly fragile as the S&P 500 stalls near elevated levels. A rebound in Treasury yields and valuation divergence around Nvidia have added pressure, raising concerns that the market is approaching a critical tipping point. Historical seasonality is adding to the caution, with September often the weakest month for U.S. equities. Michael Santoli said several key indicators are in a "taut" state and that a breach of critical thresholds could sharply alter market behavior. The market-implied probability of a September rate hike has climbed above 50%, while investors are also grappling with reduced clarity from the Fed, a two-speed economy, and yields nearing 5% that have left much of the debt issued in recent years trading at a loss.