The phrase is reportedly derived from the fact that because an upward market is called a bull market, investors "pulling in their horns" denotes a more cautious approach than rampant bullishness.
This change can either be a temporary phase, as investors re-enter the market after a price decline, or it may be the pcursor to a protracted decline. In extreme cases when asset prices have increased especially sharply and valuations are extended, the period when investors pull in their horns may be the first sign of a impending bear market.