The reason for this jump lies in the AES intruction set, which can be found with the Bulldozer but not with the Thuban CPUs from AMD.
If you are looking for some more info in the AES instruction set you can check out this LINK
which will take you to wikipedia.
In fact, you can see a simliar jump with Intel CPUs. It is exactly the same story. From the moment when the put the AES instruction set "into" the chip encryption performance increased massively.
I hope this answers you question.