It would be nice to have more clues..
Force it to happen again and be ready with a spark plug wrench. When it dies, remove the plug by the side of the road and see if it's wet with fuel. Carry a new, spare spark plug so if the plug is fouled you can get restarted easily.
Pull the head off the engine and clean the piston's top and combustion chamber of any crusty carbon deposits.
Stopping and idling for a couple minutes makes the engine hot since there is no air passing over it. If your idle speed was set while the engine was cold, this may have something to do with why it dies when you take off.
Pull the carb off and check the float level. Since, afaik, there's no air/fuel idle adjustment on that Bing carb, proper float level is critical as far as a idle adjustment. Idle speed should be set after the engine has had time to get nice and hot.
"Backfiring" could mean the spark (or something else) is igniting the fuel mix at the wrong time. Ignition and combustion should only happen with the piston near top-dead-center. Otherwise combustion gasses can travel the wrong way at the wrong time.. This is where ignition timing and adjustment and general ignition health might come into the picture.
Although the bike runs good it's got a problem. A little normal maintainance-type investigation can't hurt .. and you might catch a small problem before it becomes a bigger problem.