It was already explained in another thread, it's a security measure (of rather dubious quality) they put in for when price is falling too fast. This is the kind of "solution" that makes software engineers around the world scream out in agony.
How does this solve the price falling too fast?
It is a bandaid that Mark implemented to avoid a repeat of the hack a few years ago when someone faked a market sell all the way to zero (cleaned out the order book). It is not designed to prevent the price from "falling too fast."
(I am not condoning this "security system" BTW, just trying to get the facts straight.)
Seems like a dumb idea to create more confusion by replaying trades instead of just giving folks a message: "Trading temporarily halted due to a possible security issue."