This is what happened.
I parked my bike exposed to the sun (around 11.30am) and went for lunch. When I came back 45 minutes later, I pushed the bike and it refused to budge. It seems that the wheel got locked, I need to exert a huge amount of force before I can move the bike.
I then started the bike and ride it to a nearby shady area so that I can take a look. Although the bike can move with the engine running, it still feels like driving a car with the handbrake on. There was alot of resistance.
I have no idea what happened even after inspecting the wheels. My guess was that the brake disc expanded due to heat from the hot sun. So i sprayed some water to the braking system to "cool" it down hoping it will contract. True enough, my bike can move freely afterwards!
Anyone experienced this before?