Nightmares lead to confrontation. Confrontation leads to mystery. Mystery leads to the darkness.

After the Mane 6 all have similar nightmares of an abandoned mineshaft and their murders at the hands of Twilight Sparkle, they decide to investigate, leading them to where they confronted the Diamond Dogs before. Only the Diamond Dogs have changed, and there appears to be many more secrets hiding in the darkness of the underground caverns.

Not to mention that clanking... the continuous infernal clanking...