I would say it could be suspicious, and unfortunate that dogs died. But Debra is right, necropsies need to be performed to be sure. Dogs get into so many things without our knowledge.
On the other hand, I never use these chemical flea medicines. I tried the topical ones and they burned the fur right off my dogs. These medicines are pesticides and if you read the labels they say to be careful and not to apply to human skin - safe really? I've gone to herbal (Doc Akerman's for example, no cloves) sprays even though they need to be applied every day. My excellent rabbit hunters often come back not only with the rabbit but also the rabbit's fleas. I've seen the fleas die and drop off immediately when sprayed. The ones with citronella also keep mosquitoes away.