How residue forms
Cleaning solution is designed to hold soil in suspension so it can be extracted. If the volume of solution applied exceeds what the machine can recover, the surplus dries in place with the detergent still active in it. That dried detergent is hygroscopic and tacky. Every particle of dry soil that lands on the pile now adheres instead of sitting loose where a vacuum could lift it. The carpet effectively becomes flypaper, and the more foot traffic it sees, the faster the effect compounds.