Q: Intake manifold backfire when key switched to ON position

hi, car runs great still, except lately ive been having these backfires from the intake manifold upon switching ign key to ON position! i wouldnt say its a loud bang but its annoying enough that it spits the vacuum hoses off the manifold... except once i decided to test it while at WOT and baaangggg.. it destroyed my air intake box and chewed up the honey comb at the AFM... i have already swapped injectors + fuel regulator and tested them first for any leaks to rule out excess fuel as the culprit.... but the problem persists... im just wondering if its normal for all sprk plugs to fire up at ON switch igniting any remaining fumes there.. or is this some electrical fault im facing?

PS:this happens only after engine has been off for at least 2hrs...

your help is much appreciated as im really running out of guesses here..

cheers

My car has 240000 miles.
My car has an automatic transmission.