Seasonal Market patterns, are unlike an oscillator-example the full stochastic, this is a repeating pattern in the markets.
The fall selloff is the most common stock market pattern. Like anything else, it is not set in stone, but you will see it occur more often than not. And it has been occurring the past few weeks.
the chart linked above is many years laid one over the other, in an attempt to look for a seasonal pattern.
I have seen it said that the reason it is such a regular pattern is that October is when most mutual funds end their fiscal year, so they need to sell any positions to lock in tax losses.
The second most common time for a seasonal Nasdaq selloff is Feb-March each year.
Additional disclosure: You can't base a whole trading system around this, but its good to be aware of when the media drum beat says all is lost and then you remember how many years you have now seen this happen and its a good time to buy at a discount.