The rhythm of the market itself also leaves stocks potentially vulnerable to a bear assault. The "average stock" has been lagging the big-cap indexes badly, as most observers have been highlighting lately.
Fewer than half of all S&P 500 stocks are above their individual 50-day moving average, which means most stocks are not in a short-term uptrend.
Weaker market breadth sometimes — but far from always — precedes a deeper setback in the headline indexes.
Big Tech has faltered, too, with semiconductors and then each FANG stock pulling back significantly. Political scrutiny has coincided with an the outsized weighting of tech in the market that has tested investors' appetite for the sector.
And emerging markets sagged last week — making their recent rally look like mere bounces within oppressive downtrends rather than decisive recoveries.
And with the recent softness in the S&P 500, the index now sits less than half a percent above the 2,872 level of the January high, which capped the market for seven months. A further pullback below that threshold will no doubt get the bears growling about a possible "false breakout" or "double top" — cosmetically reminiscent of the market peaks in 2000 and 2007, when an early-year record high was approached or barely exceeded several months later before the indexes buckled into a bear market.
October also means a suspension of most corporate-buyback programs around earnings season, which likewise could embolden the bears. While I've argued that heavy buyback activity was more a background factor and psychological crutch for investors rather than a direct driver of stock prices, it's also true that some patients get sick right around the time they stop taking a placebo.
One skeptical observation of 2018 is just how much profoundly good news it has taken to muscle the S&P 500 higher by "only" 8 percent: 20 percent profit growth, 3.8 percent unemployment, a 3-4 percent GDP pace, and some $1.2 trillion in annualized cash sent to shareholders this year via dividends and stock repurchases.
What happens when the news gets slightly less great?
So there's the bear buffet laid out for October, a month already living up — in a modest way — to its reputation for increased volatility.