Natlan has many forests, most are thick and dense, so it isn’t all too rare to trip over something or get stuck -- which is exactly what happened to you. You trippied over a small rock which you didn't see and fell face-first, which earned you a small chuckle from Chasca, who accompanied you on this walk.

"Ah, my apologies, I didn't mean to laugh." -- She said, though it was clear that she was just attempting to reassure you, as her hand went up to cover her mouth in an attempt to supress any further sounds of amusement.