The answer would be one of our least favorite scenarios... Colic.
Even if they didn't cause colic, worms are yucky. God even stuck some in Hades. In adult horses, small strongyles are most often
incriminated while living, hibernating, and emerging from the large intestinal walls. Tapeworms affect young and old; they tend to hang
around the junction between the small and large intestines, creating problems which range from very mild to fatal in severity. The
youngsters have more than their fair share of roundworm (ascarid) problems (most older horses become somewhat immune to them), and
these typically cause impaction colics and general unthriftiness.
The teeth association was interesting: it's been found that a significant percentage of horses that were treated or operated on for
impaction colics had sharp molar points. Perhaps they did not chew their food up as well due to discomfort.
Ice, frozen water tanks, and even cold, cold water seem to discourage water intake. Horses just don't drink as much water in the winter. Couple that with increased dry matter content of forage intake (we tend to feed more hay, and good pasture is scarce), and you may understand why we see more impaction colics when it's cold outside.
Ice-free, automatic waterers are a great investment; tank heaters are okay, but make sure they don't short out and shock your horses. Remember that good quality hay is just as important; poor-quality, "stemmy", and poorly-digestible hay can cause impactions as well.