anyone knows why this is so?

It's condensation.

The glass is at room temperature and the juice is cold. The juice is colder than the air around it. The air around the juice can't contain all the humidity. Colder air cannot contain as much fluid as warmer air. So the water condenses out of the air around it and appears on the outside of the glass.

Hope this makes sense.

This is the process of condensation. (See link below) Water that was before in vapor form condenses along the surface of a glass which is cold enough to make the difference between the solid and the gas form of water.

Air contains moisture which is ordinarily in the vapor form. The amount of water vapor the air can hold, is temperature dependent. When this vapor comes in contact with the exterior of the cold , air is cooled down and can not hold the same amount of water vapor. The excess moisture depending on the temperature of the exterior of the glass settles down on the surface in the form of liquid.