Why did my dog have a seizure?

I have a 12 yr. old terrier mix. Found out like 6 months ago she has diabetes. I have to give her insulin two times a day every 12 hours, i give her 15 units each time, She started off with 6 units 6 months ago and it did no good cause she was still peeing (and drinking a ton of water) all over the house, so she finally got under control at 30 units total a day, which the vet said was totally fine, and also said she was capable of having seizures sometime down the road from the insulin; but she's been having 30 units a day every 12 hours for the last 3 months I would say, why would i come home randomly at 4 in the afternoon and find her having a seizure? (yes she's fine now, I called the vet freaking out not knowing what to do he said to put corn syrup on her gums and she should be fine which she was) cause she went from shaking horribly and not being able to stand up and laying on the ground with her head jerking back and fourth. I was scared horribly for her. That's my question.