Carpet is threads glued to a porous backing. Spray liquid on it, the liquid will penetrate to soak into the foam padding underneath.
You were probably squeezing out the excess liquid in the padding.
If you really want to clean it. Then you need to remove it. Take it to the car wash. Flip it over, work from the back side. Then hang it outside to drain and dry. The padding probably needs to be replaced. Not sure if it can be cleaned.
As an alternate, you could try a Rug Doctor carpet shampoo machine when the carpet is out of the truck.
Good Luck.