A bad radiator cap could cause the coolant to start bubbling but most commonly the cause of coolant bubbling is due to a faulty thermostat.
A faulty thermostat that is causing sporadic opening and closing of the thermostat can cause the coolant to churn and then start to boil.
It can also cause pounding noises in the radiator too.
If the radiator cap doesn't seal properly then it can cause overheating of the car or trucks engine because it cannot build up enough pressure to keep the coolant flowing through the engine and radiator properly to cool off the engine.
So you can replace your radiator cap if you think it's going bad as it's a pretty cheap and easy fix but if the coolant is still bubbling then it likely is your thermostat going bad.