What triggers Christ's second coming?

That statement is vague; I think we have a more definitive sign given by Christ himself: when you see the man of sin sitting in the Holy Place, calling himself God, doing miracles, and holding the whole world under his control, that is the major sign, 2 Thessalonians 2, that Christ is at the very door. God gives them strong delusion because they refused to love the truth, so this man of sin locks up their deception with false miracles. We have another clue in Matthew 24:32, the fig tree in leaf, which if it represents anything is national Israel: since 1948 we have the nation of Israel, a fig tree in leaf but not in fruit, meaning they are no closer to Christ than in the first century. How long from that, who knows, but it will not be that long. There are practical signs too, like population growth accelerating toward sixteen and then thirty-two billion, which this world cannot sustain.