Carpet brown out can happen when cleaning natural fibers like cotton. Browning is typically noticed the next day after a spot is cleaned. It will usually look like a brown ring on the outer edge of where the spot was or and entirely new solid brown spot. Carpet dries from the ends or tips of the fibers (where the air meets the fiber) as the solution evaporates from the tips, the water rises to the surface bringing with it everything dissolved in it, and then the water evaporates leaving everything else behind. This leaves on the tips soil and residue plus anything else left in the carpet such as high shampoo levels, cola, coffee, etc. This upward flow of moisture on fiber surfaces during drying is referred to as wicking.
The factors contributing to browning are slow drying and over wetting. Also, if the fibers consist of cellulosic material like jute or cotton. If you want to see a quick example, poor a little water on a piece of paper and let it dry. To help prevent this wicking action many times direct air through fans or air movers needs to be applied to the spot. There are also commercial products available to neutralize carpet browning.