@dadoje:
Thanks for all the responses. We don't think it's a reaction to bites or plants because it's happenned twice a year a part. If it happened more often if might be easier to figure out not that I want it to.
Was it truly 1 year apart so that the incident happened at the same time of year each time? If so, it may be an environmental allergy where they bug or plant is only active at that time of year. I have heard of dogs only having a reaction to something once a year at the same time each year. These incidents occur in Southern California. They are always in the late spring/early summer and the dogs break out in hives all over their bodies. No one knows what they are reacting to but it does seem to be some sort of allergen.
I have heard of other dog owners in mostly the midwest who have had dogs that have collapsed much the same way you describe. I don't know if any of them had dogs who had a similar episode again at a later date.