Cannae was indeed a roman disaster, if you look at all the historic battles, this battle was surely the one with most casuals. If im not mistaken died there over 70000 romans and around 5000 Carthaginians.

There are some different thoughts about how the battles was fought, the majority claims today, that the drop back in the center of Hannibal's center was intended from start.
His cav got in from the back after some skirmishes vs. the roman cav. Thats it.
The romans was so pressed into each other, that they wasnt able to move hands or swords. It was a 6 hour lasting massacre, it wasnt jsut a loss, this was a splatter killing monster massacre!

Well, hannibal won some battles, but lost the war. Still, this battle was one of the biggest mess-ups in roman history.