A swollen upper radiator hose is mostly caused by overheating of the engine or too much pressure in the radiator hose caused by either a clogged radiator or other clog in the coolant system.
If your engine gets too over due to overheating it can cause the coolant to get super hot which can lead to the upper radiator hose swelling.
If it's not that then your radiator hose could be going bad and becoming too soft and is swelling out under the normal radiator and coolant pressure.
If that's the case then you need to replace the radiator hose before it bursts.
A good radiator hose should feel firm and not too soft or have any soft spots in the hose and if it doesn't feel firm or has a soft spot then you should replace the radiator hose.
Radiator hoses should be replaced every 5 to 10 years or 100,000 to 150,000 miles.