I thought this was a joke. I've found only one site - CBS Boston - which says anything about a substance affecting the brake light switches, -
" Subaru says cleaning products containing silicone can emit a gas that can seep into the brake lamp switch and cut off the electrical contact. That can stop the brake lights from working but brakes will still function. "
other sites mention the recall but not the reason. The CBS Boston site also says there have been only 33 reports to Subaru in the US. I wonder what concentration of substance is needed to cause a problem, perhaps it builds up slowly.
Still sounds odd and nothing on the Subaru site.
Dave