Rare Wall Street sell ratings picked up just before this market began to crack

Chalk one up for Wall Street analysts.

The group known for being overly-bullish lemmings, got bearish in the last month ahead of the crack in the markets this week.

On average, about 4.5 percent of newly issued ratings for S&P 500 companies in the last year were sells, but in the last month that bumped up to a little over 5 percent, according to a CNBC analysis of the thousands of ratings in FactSet's broker database.

Sell and underweight ratings are very rare -- the vast majority of ratings for companies in the S&P 500 at any given time are buys or holds. In any given 30-day period, only a few hundred sell ratings are issued out of thousands of broker ratings. That means that it only takes a few dozen additional sells to move the needle substantially.

The half a percentage point movement in negative ratings we've seen in the last month is a relative change in sells of about 16 percent overall. That could be a temporary blip that disappears next month or the start of a wave of negative equity sentiment.

The number of buy ratings for S&P 500 companies has shown a loss of a few percentage points over the last year, from around 47 percent to 45 percent. On a quarterly basis, sell ratings are also up and buy ratings are also down compared to the same period last year. Those slipping ratings seem to be taking place across the S&P 500, but are especially concentrated in retail and financials.

As the market headed into the close on Tuesday, stocks were on track to have their worst day of the year and major indexes are down for the month. In a recent Bank of America Merrill Lynch survey of fund managers, more respondents said equities are overvalued than any time in the last 17 years. It could be that brokers are starting to feel that same way.