There are many reasons why an engine should spit back through the carb. I once encountered a Ford that had a worn exhaust lobe on the camshaft so the valve was barely opening - the engine ran very well until the throttle was opened. It is still a huge mystery how the owner had managed to drive it to my workshop.

The most common reason is a lean air/fuel mixture caused by one or more carb jets being blocked.

A thorough carb clean is a sizeable task that shouldn't be undertaken lightly and it is best practice to tackle it last after ensuring everything else is as the manufacturer intended it to be - from spark plugs and spark power/quality, timing, valve operation, fuel delivery quantity and pressure, etc., etc.