Aries: They forgot to tell you something earlier and just now remembered 

Taurus: They want to talk about life and the universe

Gemini: They need to talk about something serious 

Cancer: They're lonely

Leo: They need help with something

Virgo: They're bored

Libra: No one else is texting them

Scorpio: They forgot to do their homework and now is asking you for answers

Sagittarius: They need to confess something

Capricorn: They could not sleep well

Aquarius: To check if you're still awake or not

Pisces: because they love you and want to check if you're doing ok