More important is why they do it: it's a chance for the market to pause and rationally assess automated trades. The reason we're seeing it more often is because

More important is why they do it: it's a chance for the market to pause and rationally assess automated trades. The reason we're seeing it more often is because it's a reaction to stuff like "flash crashes", which weren't a thing (as much of a thing, anyway) before bots took over the market. Of course, journalism remains trapped in a death cycle of click-chasing stupidity. This means that the triggering of the mechanism itself has now become a source of hype to propagate the panic ("OMGOMGOMG the market fell so fast that it tripped the circuit breakers!! Panic! Flee! Run away!"), so it's less clear that the mechanism is a net good.