Noted naval aviation author Barrett Tillman cites a problem with the oxygen rebreathing system in Best's SBD as the cause of the generation of caustic soda fumes (sodium hydroxide):

Other casualties survived but were lost to the cause. The most notable was Dick Best. Back in Pearl he was examined by the senior medical officer...who determined that the SBD's oxygen rebreather had become heated during the unusually long search on the 4th. The heat created gases that turned to caustic soda, activating latent tuberculosis. It led to permanent grounding.

James D'Angelo states the following, drawing from Best's military medical records:

Just after Richard Best landed on Enterprise, he began to cough up blood. While flying on his mission in the morning, a faulty oxygen bottle expelled caustic soda (sodium hydroxide) into his lungs. Over the next twenty-four hours, his hemoptysis (coughing up blood) continued, and he became acutely ill with a temperature of 103 degrees. He was admitted to Pearl Harbor Hospital on June 5, 1942, where his chest film revealed an infiltrate in the right upper lobe and increased mottling extending out from the hilus (the triangular location of each lung whereby the pulmonary artery and vein, lymphatics, and bronchial tube enter and leave each lung) to the periphery of the lungs. Stomach washings and sputum analysis revealed the tuberculosis organism. He was transferred to Fitzsimons General Hospital in Aurora, Colorado, where he received proper treatment for his tuberculosis. Best was hospitalized in Fitzsimons Hospital until September 1943.

Best, sometime in the past, had contracted tuberculosis, which remained in his lungs in an inactive state for years. His accident in the morning with the faulty oxygen bottle permitted the inhaled caustic soda to erode away a tuberculosis granuloma (a benign nodule of chronic inflammatory tissue) which contains an inactive form of the tuberculosis organism. The erosion of the granuloma by the caustic soda transformed the inactive form of the organism into an active form. In addition, the caustic soda caused an aspiration pneumonia. June 4, 1942, was the last day Best would fly for the U.S. Navy.

The SBD's rebreathing system was mounted in the crew compartment and consisted of canisters which contained the purifying material (for the pilot, two canisters mounted to the right of his seat, and for the gunner, one canister mounted next to the oxygen bottle and another next to the tailpipe valve), a small oxygen bottle located in the rear right of the gunner's compartment, and face masks that the users wore. It was used to remove the carbon dioxide from a user's exhaled breath, allow them to re-breathe some of the oxygen that was not used, and supply a small amount of oxygen in addition to oxygen supplied from the main cylinder. The material used in the rebreather was usually sodium hydroxide, or some other material which absorbed carbon dioxide. If the device containing this material (usually a pad covered with the substance) was abnormally heated or came into contact with water, it could release caustic fumes through the mask.

Sources:

D'Angelo, James. Victory at Midway: The Battle That Changed the Course of World War II. Jefferson: McFarland & Company, 2018.

Tillman, Barrett. Enterprise: America's Fightingest Ship and the Men Who Helped Win World War II. New York City: Simon and Schuster, 2013.

United States. Navy Department. Pilot's Flight Operating Instructions, Navy Model SBD-6 Airplanes. Washington, D.C.: United States Government Printing Office, 1944.

I believe caustic soda was used to clean impurities out of the lines which were used when the oxygen tank was hooked to the pilots mask. Caustic soda was once used commercially to clean pipes and tubing. I suspect when they cleaned the oxygen tanks or the lines connecting to the tanks to the mask they either used too much of it or forgot to clean it out? This is the best answer I could come up with.