Google says:
Once a wall to wall carpet or rug is installed in your home and as it is trafficked upon pressure is placed on the pile yarn. On a dense piled carpet the higher density holds the tufts of yarn close to each other. On a sparser construction there is not a great deal of support between the joining rows of yarn tufts and some of the tufts push over while the others remain erect and the corn rowing appearance is created. The corn rowing usually develops in the traffic areas as these are the areas that get the greatest foot traffic.
Identifying Corn Rowing
Carpet Construction: Usually has a wide stitch rate (wider distance between rows of tufts) and/or the carpet has a higher pile such as a shag or loosely constructed frieze.
Where it Develops: Corn rowing most commonly develops in the traffic lanes, at pivot areas or under doors that drag on the carpet.
More Common Causes
Vacuuming with a very aggressive vacuum or with the brush set too low on the vacuum. Brushes should be adjusted to touch the tips of the carpets pile.
Improperly installed carpet that has not been stretched tight enough.
Improper carpet pad (cushion) that is too thick or too soft. Carpet cushion should be no thicker than 7/16″ with a minimum density of 6-8 pounds.
Poor maintenance such as in frequent vacuuming or always vacuuming in the same direction.
Doors rubbing on the carpet create the corn rowing condition in kind of a pie shape.
Preventing and Correcting Corn Rowing
On longer shags and loosely constructed cut pile carpet you can regularly rake the carpet with a carpet rake that can be purchased from a janitorial supply house.
Regularly change direction when vacuuming.
Do not set the brushes on your vacuum too low, only allow them to touch the surface of the pile.
Carpet has a memory and once corn rowing sets in you may not be able to permanently correct it. Raking regularly may be the best you can do.
Manufacturer Related Corn Row Appearance
There is a similar appearing manufacturer related condition to corn rowing that can occur with cut pile carpet. This manufacturer related corn rowing is actually gauge lines. Gauge lines are fine lines that run lengthwise and always the same distance apart from 1/4″ to 2″ and they give the carpet a corduroy affect.