see here.....
http://www.trialscentral.com/forums/topic/23670-beta-evo-4t-250300/page__hl__evo4t__st__30
Nothing to worry about - common with large capacity 'thumpers' and twins.
Its the ignition of excess unburnt fuel introduced when doing what you describe (and sometimes made worse by an air leak into the exhaust system - like if you had the system apart then didn't seal it properly)
My BMW R1150GS was a bugger for it - scared the bejeesus out of many a blue rinse waiting at the bus stop.
Found one to watch with this bike though. The engine breather exits into the bottom of the air box. After a big off and the bike being upside down - then going down a steep hill, my bike began popping and farting all over the place. I later discovered the engine had spat some oil out of the breather into the bottom of the airbox and this must have then been pulled / poured through the carb when going downhill causing the popping and banging. Worth checking if ever you have a big off.