Hmm, no answer on the actual cause, but one thing that jumped out at me is that water doesn't contain the beneficial bacteria. As long as you keep the surfaces damp you can change all the water in the tank with no loss of the bacteria because they live on surfaces. So this habit of washing everything may be an indirect factor. If you are washing the items in anything but tank water or declorinated water, or scrubbing them to vigorously you may be causing a mini cycle each time you do a more thorough clean of the tanks.
It is wise to clean, but unless there is algae build up or a thick layer that comes off easily the surfaces other than the viewing windows should be left alone. Siphon out as much waste as you can when you change water, or even whenever you see it, but don't scrub stuff sparkling clean.
Sometimes the biofilm does build up to levels beyond our liking, and it is ok to scrub some of that off, but be sure you don't also mess with the filter at the same time, so that you don't upset the system to much. Change filter pads or do a little heavier cleaning but not both at the same time because that helps keep some surfaces undisturbed so the beneficial bacteria remain in sufficient numbers for your tank's needs.
Also stress is somewhat cumulative. For a fish having the water level go down to where their fins stick out slightly once or twice a week in tank is less stressful than being netted constantly. The fish may be fine most of the time, but you never know when the cumulative and an additional scrape by the net allows such an attack as seen in your troubled betta to get started.
