When asked last week on CNBC's "Futures Now" what explains the trend, Kosar demurred.
"When you're looking at the fourth quarter, you look for things that stick out," he said. "You don't necessarily know why, but this is kind of one of those things."
Read More Dollar rally presents 'rare' short opportunity: Trader
Still, not everyone is getting nervous.
"Just to bet on something without any fundamental justification behind it? I'd rather rely on studying chicken entrails or my Ouija board," said Chicago-based trader Scott Nations.
In fact, any trade based on the factoid would have to be an especially quick one. After next week, December — the strongest month of the year per Kosar — isn't far behind. During that month, the S&P tends to rise by 1.5 percent on average.
"So look for possible lows to be made next week and then after that in terms of seasonality, it looks like it's going to be blue sky," the technician said.