When the electron transport chain is backed up, electrons can flow backward through Complex I, generating NADH.

“eventually there will be so much nadh and so little nad plus that there's nowhere for those electrons to go and they will end up jumping onto an oxygen molecule and produce superoxide which is a reactive oxygen reactive oxygen species and it doesn't do it until everything is set up to say we really don't want any more metabolic substrate coming down the electron transport chain so you've got to reduce all of the nadh that's the nadh in the mitochondria matrix and you've got to have a high membrane voltage and you've got to have a low demand for atp once those things are all set up then the cells will generate the mitochondria complex one will generate reactive oxygen species”