Liquid dripping from 777 wing
 

On a recent flight from LAX, passengers observed liquid dripping down the lowered flaps between the pylons of the starboard wing of 777-300.
The liquid formed a 1 meter wide band of steady drips until takeoff.
A rough estimate of 2 liters a minute for 15 minutes.

It was noticed by passengers seated in rear section, after pushback on an evening departure, humidity was around 65%.
No other moisture was visible on the airframe.
Cabin crew informed flight crew who strolled down the aisle and had a look for themselves, they later informed passengers that it was condensation.

Cabin crew had not seen the effect before.
A freak set of weather circumstances?

It is quite common. I have seen the lower section of a wing with a formidable layer of frost/ice, esp between the pylon and wingroot.

Thin layer of ice usually forms on the under surface of the wing between the engine and the wing root due to cold fuel which cold soaks the wing.

Definition:Cold soak Can occur in ambient temperatures between −2°C and at least +15°C, ice or frost can form in the presence of visible moisture or high humidity if the aircraft structure remains at 0°C or below.

What you were most likely seeing is the melting ice dripping away. Sometimes On walk arounds in higher temperatures it looks like it raining down there while inspecting the gear.

Yep.

During a flight, the wings and fuel in them can get down to -30 degrees centigrade or lower, (the temperature of the air at cruising levels is around -56).

After the aircraft has landed, the wing itself warms up quite quickly, but where there is cold soaked fuel remaining in the tanks, it causes moisture in the air to condense out, quite often forming ice. During the walk around of an aircraft that has been in the cruise a long time, there is often water dripping underneath from this ice melting as the wing warms up and (relatively) warm fuel is pumped into the tanks.

The picture of BA38 (777) on low approach just before its crash-landing at Heathrow famously showed frost on the underside of the wings from the cold-soaked fuel tanks.

Had it made the runway safely, no doubt that frost would have melted and dripped off the wings, as well.

Conversely, on arrival in humid San Juan once, the humidity condensed on the entire cold-soaked airframe, fogging the cabin windows during taxi-in.