Sources of volatile hydroxils include fermentation and plant respiration. The authors did not say "they have no idea", they said they do yet have a mechanistic explanation. They clearly have some ideas and those ideas are related to the differences between the tropics and the rest of the planet and they are all calling for more studies on specifically this.
"However, the authors do not yet have a mechanistic explanation for the last decade's global changes in hydroxyl concentrations. Future studies are needed to investigate this further, Frankenberg says. The researchers also would like to see the trends they detected verified with a more detailed study of both methane sources and sinks."
Space is to place as eternity is to time. -- Joseph Joubert