Water quality and impurities are a huge problem, and are usually the last thing to be checked.
Tony Martin was nearly closed as his new stable complex was built with pyrite contaminated concrete that poisoned his water system, causing him to lose Giggi among other high profile clients.
Deep well drilling in Saudi Arabia brought up water high in zinc that caused copper deficiency in horses and dairy cows over there; there was a similar problem in Co Meath on a farm purchased , ironically enough by a Canadian mining contractor Gene Kruger to get away from mining became the site of Bula Mines, named after the Champion Hurdler.
As for the myriad of causes of colic, you name it and it could cause it.
Diet, diet change (either by feeder or feed company ) , exercise change, transit, medication especially antibiotics that can change the gut flora.
Anything that can change the gut motility or cause gas production the animal cannot handle.
Of course where colic strikes laminitis can be the next problem to contend with.
WPM made a very interesting observation during the Festival, that when the stable star goes wrong everyone else equine in the yard can follow suit.
When he gets his daily equine injury list he scans it to make sure the big names are absent before continuing his breakfast in peace.