It's a question of velocity - markets go up and down all the time... but speed breaks to prevent the market from going down to fast also need to prevent the reverse case - some sudden inexplicable and extreme price spiking.
Basically - we should protect against any really dramatic sudden changes as chances are that something is wrong.
In the last year the S&P grew ~22% prior to this recent drop, that's fine... and on a day swings in the low single digits might be fine. But if we woke up tomorrow and the S&P had recovered the pre-drop value and then grown 20% over that price - something weird is clearly going on and we should be worried.
Basically - we should protect against any really dramatic sudden changes as chances are that something is wrong.
In the last year the S&P grew ~22% prior to this recent drop, that's fine... and on a day swings in the low single digits might be fine. But if we woke up tomorrow and the S&P had recovered the pre-drop value and then grown 20% over that price - something weird is clearly going on and we should be worried.