The exterior sheathing was designed as an air infiltration barrier (non-ventilated wall pressure envelope), however with sheathing joints not ending over the stud framing the exterior air was allowed to flow into the wall cavity causing it to ventilate.
With large open holes under the inside a/c unit, those caused during the installation, the home is drawing in outside air contaminates. Additionally with pressure loss of ducts to the crawlspace, outside air is drawn into the home due to photo like above shown (poor air barriers).
Note the aluminum ductwork as having corrosion from open exposure to the crawlspace. It is not known who cut open this bottom board material (pressure envelope) to expose the floor, however the a/c duct loss to the outside of the home causes the home to operate under severe negative pressure that pulls into the wall cavity unwanted air infiltration and moisture.
Being the bottom board is open allowing air infiltration, note the water staining of the duct from condensation, as warmer ground moisture collides with the 45-50 degree cooled duct. As well note poorly taped connections of the background floor plenum of the air handler - this duct work shown evidence of poorly sealed connections allowing considerable air leakage, an unbalance of pressure adding to further condensation concerns.