Coolant hoses on Carburator

[Jules-C]

Keeping the engine running keeps a flow of cold air flowing through the carb over the small ice crystals allowing them to grow, hence the problem getting worse and worse as you continue to ride. Stopping the engine means the heat from the cylinder head can set up convection current in inlet tract allowing slightly warmer air to reach the carbs and melt the ice crystals.

Yes it doesn't happen often and never if you have bike garaged, even a cover over bike makes a big difference. 

But if it wasn't a problem they wouldn't have tried to solve it with the carb heaters.