We have more melanin. Good thing overall, but it also means our skin reacts faster to sun, acne and irritation by producing extra pigment.
Common triggers:
Sun exposure (biggest culprit, always)
Acne marks (post-inflammatory hyperpigmentation)
Hormonal changes (melasma)
Heat + friction (even from masks or dupattas sometimes)
And once these spots settle in, they take their own time leaving.