i was told when i was a nub here that it was from either exelent growing conditions, or high humidity under the canopy. both have good explanations.
yet in my growing ive come to see them as a sign that the plant is nearing a rootbound nature, or a bug eating the roots. this is only based off of 150 or so plants ive grown in 8 months or so(males/culled etc included).
i was told it is the stem trying to root out into the air because it is moist/shaded. but it seems to me they become much more prominent when the plant is having problems growing/sustaining/expanding it's root system.
just my opinion im not sure.