You say it was spring, so it might have been that they all got too much rich grass at that time. It could have also been due to them suddenly getting much more out of whatever their normal feed was, due to reducing the worm burden. But yes, it could also have been the wormer. Can't recall whether I've heard of this specific reaction before, but there are various 'side effects' to wormers, as there are to all drugs. Especially as wormers effect the digestive system it's quite conceivable they could cause a metabolic upset.
Ivermectin is quite a powerful chemical too. I've read studies that show bugs(worms, dung beetles etc) won't touch Ivermectined poo for over a year.... so nearly as strong as McDonalds preservatives!
As I do keep my horses on a property where there are others coming & going, not to mention the kangaroos & wombats visiting, I do use a chemical wormer at least once a year on average. I use Ivermectin, ensure their nutrition is good, poo is harrowed(well, more often kicked around!
) and examine poos or occasionally do fecal counts, and use a different, less toxic wormer occasionally if required.
I don't think people generally appreciate how common laminitic bouts are. If the horse is overall healthy, healthy diet etc, a laminitic 'attack' is usually short lived and often apparently painless - or at least not noticed at the time. If it is only temporary and the hooves/horse is well managed, it doesn't seem to do any lasting or serious damage to the feet. Therefore I'm not overly concerned with these sorts of issues. I would however be vigilant and take note of anything of the sort, do things that cause it as seldom & short term as possible, change my ways if the horse had longer or more frequent 'attacks'.