Combination of "fap", meaning to masturbate, and "apocalypse".
When so many people are simultaneously masturbating that it brings about the apocalypse.
God is supposedly omnipresent, which means he sees all, all the time. Fapture occurs when there are so many people simultaneously masturbating that God can no longer stand to see it all and comes back to tell everyone to knock it the hell off.
Did you see Jessica last night? She's so hot I swear she'll cause the fapocalypse.