It can be related to temperature drops and changes in the barometric pressure along with a slew of other things. You have to remember, Colic is sort of like Navicular where it's a general term, in this case... It means belly ache. And as we know, a belly ache can be mild to severe to life threatening with horses.
I have one that doesn't drink the cold water so when it gets really cold out, I haul luke warm water down from the house for him. As of now, it's just the one that I have to do it with.
There are a myriad of reasons for why horses colic when the temps drop, while other horses are fine. We can't just point to one thing and say, 'That's it! We've found the smoking gun!' because it simply doesn't exist.
Dehydration, stress, nervous temperament, all can play a part in why some horses colic when the weather changes. It's not so much the weather getting cold as the temperature fluctuations that occur this time of year. The mornings are chilly and the afternoons are warm, sometimes even hot. Horses with delicate digestive systems can easily get sick when the temps seesaw up and down.