Changeable weather conditions, cold weather meaning horses don’t drink enough, people keeping them in because of the rain all of a sudden so change of routine, emergence of encysted small redworm. Or just coincidence and bad luck- my OH is a vet and they usually do come in waves
A very wet winter and a lot of reduced/restricted turnout and horses standing in a lot would be a likely factor at a guess. Lack of movement is a massive contributor to colic at this time if year.
A lot of people feeding straw too due to hay prices/lack of availability and not all horses will tolerate it, or it needs to be introduced slowly.