sometimes it could be certain medicines, foods, flea/tick meds, etc or it may be a chronic condition or something that is just now showing up, how old is the dog? it could also be something in the compost, something he/she ate, medicines/poisons (many chemical cleaners, common everyday products/cleaners, laundry detergent, chocolate, foods etc are toxic to dogs and can cause problems if ingested), stress, etc. Is this the first time this has happened? How long did it last? was it sudden (was he fine before and then all of a sudden wham, or were there symptoms even subtle symptoms)? any other signs of illness? any other pets? if so any other sick pets? anything the dog could have gotten into? any chance the dog got loose and got into something off the property and/or got into something on your property without you noticing? has the dog been sick lately or was it just sudden? any rat poison, vermin control, toxins, anything else you can think of that the dog may have gotten into? is it just this dog or others too? How old is the dog? some dogs/animals can also have disorders that can cause them to seize like some people do dogs can get it too, they can get a lot of things we can like epilepsy and diabetes and stuff too but there are medicines now to help treat and control these disorders and the dog can go on living a happy healthy relatively normal life on meds and occassionally getting check ups so even if the dog has a disorder or something there are medicines and treatments and stuff now so that the dog can still go on living a relatively normal and painfree life it doesn't affect them as much once controled so either way you need to have the dog checked out by a licensed vetinarian as it could could potenially be very serious possibly even life threatening for the dog and it could come back so you need to get the dog checked out by a licensed vet and treated ASAP, good luck.