Yeah, they're very normal.

There is a system on the bike called Air Induction System (I think) or AIS.

Basically, when the engine is on overrun, it opens a small valve letting COLD air directly into the HOT exhaust. Any unburnt fuel in the exhaust will get burnt up pretty much unstantly causing the back firing through the exhaust.

What a lot of people do to prevent this, like Tony says, is to block the AIS pipe from the airbox prevent the air getting into the exhaust.

__________________
09 XT660X:

CCC Stainless Rounds, Stage 1 DNA, Stage 2 DNA, PCV With Custom Autotune Map, PCV AutoTune, Home made O2 eliminator, Self machined Fork Spacers, DIY Autotune Switch.