Why blanched vegetables sometimes turn brown

Why do blanched vegetables (broccoli) sometimes turn brown? I found this answer on apartment therapy.

The most likely reason our veggies turned brown is because we covered the pot
with a lid after adding the vegetables. Yes, covering the pot certainly seems
like the logical thing to do! After all, a covered pot conserves energy, brings
the water back up to a boil, and keeps the steam inside the pot--all things that
would theoretically help our veggies cook more quickly. Here's what is really
going on in that pot: As the veggies hit the boiling water, volatile acids are
released into the water and are carried away in the steam. When the pot is
covered, the steam and the acids it contains are forced back into the water.
Once there, the acids react with the chlorophyll in the vegetables, turning them
an unsightly shade of brown. A similar reaction will take place if there's too
little water in the pot (thus concentrating the acids in the water) or if you
overcook the vegetables (thus prolonging the exposure time to the acids).
Solutions? Use a large amount of boiling water for blanching, test the
vegetables frequently to check their doneness, and leave the pot uncovered!