I guess technically if a lot of stocks went paid their dividend on the same day (went ex-divvie) you could get a 0.5-1% fall in the stock prices (depending on the dividend yield at the time) without their being a slump - the value of those dividends which have now been paid out is simply no longer part of the market. But I agree wholeheartedly with the sentiment.

I don't know if I agree with this. Suppose the stock market is driven by runaway herd behavior. If that's the case, then an inexplicably bad random perturbation might have cascading effects. Saying that the initial slump in the market is driving further decline seems accurate to me.