Juggernauts naturally hard counter tanks by virtue of how juggernauts operate.
They're immobile killing machines with incredibly high tank stats and base damages (and many times even great scaling along with those bases). Their main weakness is kiting.
Tanks are CC-heavy super durable machines with decent bases and low scalings. Their main weakness is simply damage as they rely on other champions to do damage. Their bases are typically high enough to pose some threat to squishies.
With those definitions in mind, juggernauts naturally and logically counter tanks. Not sure how you got to the point as malphite where you were three kills up on the darius but I'm willing to bet a very pretty penny that your jungler had something to do with it (unless that darius was really, really, REALLY, fucking braindead). Then when you decided that you were "ahead enough" to 1v1 darius, he whooped your ass to next tuesday because darius's kit counters malphites kit incredibly fucking hard due to his true damage ult and free armor pen.
Cleaver had nothing to do with this, it's an item that has been around for a while and only lends itself to being troublesome when ranged champions abuse it (fuck you riot for not making cleaver melee-only or at least making it half effective when operated by ranged users).