“so the runaway afterburner could not do any damage”

A bit pedantic but do you mean runaway engine, as reheat can’t ‘speed up’. Reheat can overfuel but that would require a pilot selection of RH.

So a case of Batts OFF to early then!

Okay, ‘runaway engine’ then, but you can’t have the afterburner on if the engine is not running, so that’s pretty obvious! Yes indeed, ‘batts off too early’ appeared to be the cause.