Considering colic is the #1 all-time leading cause of horse death, the number there doesn't really surprise me.
Sometimes things just come in one big group, just like at our house. We lost 3 horses in 5 months. 2 of them were old and it was their time, the other was young but developed a rare inflammation in his intestines and had to be put down. It had been years before that the last time we lost a horse.
Just to be on the safe side, I would double check everything; feeding practices, safety of the turnout paddock, safety of the stall, etc. Beyond that, there just isn't much to be done.