Colic has varying sources, so it's not just one thing. Some horses colic for what appears to be absolutely no reason at all, while some will colic when the weather changes, their feed changes, they're not getting enough roughage, they're getting too much roughage, or they're not drinking enough water.
There isn't one culprit you can really pinpoint, as it's caused by different things going on inside the horse's body, and can also be related to stress and mental condition as well as outside sources.
Don't give pain relievers before the vet gets there, as they'll mask symptoms. If the horse will stand or lay quietly, let them. If they want to roll and thrash around, get them on their feet and walk them.
There are many causes for colic. Gas colic, sand colic, ect. It has different causes, depending on what kind of colic the horse has. If your horse will just stand up, and stay calm, let him. Keep him walking. That will help his intestens move. If your horse lays down, or attempts to, don't LET HIM. If the colic is bad enough, he will not be able to get up.(: Hope I helped! Let me know if you need more info!