2006 Sienna overheating - coolant reservoir cap popped open

2006 LE 106K miles.
started overheating today, opened the hood to find that the reservoir cap had blown open and coolant had splashed everywhere on the engine.

Refilled the coolant reservoir and test drive the van, it started heating up quickly again.
did not see any coolant leak as the coolant level in the reservoir remained stable.

• Both radiator fans are working.
• Timing belt and water pump are not yet replaced. Planning to do it soon.

What could be the problem?
Is it the water pump impeller or could it be a leaking/clogged radiator?

Thanks in advance for your help.

Mike.