My dad is 99 yrs old and for the first time ever he had a mild seizure while having a bowel movement which he hadn't had in 4 days. I called the paramedics and they said it was due to his vagus nerve being irritated from the hard bowels. It didn't last long and he was back to normal within seconds and still doing fine. All his vitals checked out good so he didn't go to hospital but I'm still concerned.
You say "This hasn't happened in four days". I am assuming you mean he had not moved his bowels in that amount of time.
There are many reasons for a mild seizure, and my mother had one after a fall, a visit to ER, and coming home. She was attempting to sit up and just went into a mild and brief seizure, which did not recur.
You will recognize, however, at this advanced age, that anything may happen. Glad this seems to be a one-off. Attempt to give him stool softeners so that stools will be easier to pass.
My late husband too had seizures while pooping occasionally, but his seizures were triggered by pain(one of several things that triggered them), so if he was constipated or if his hemorrhoid's were bad that would cause him to seize.
But my husband's seizures started about a year after he had a massive stroke at the age of 48. His neurologist said that his seizures were caused from the scar tissue from all the damage that was done to his brain, and my husband continued to have seizures until he died at the age of 72 even while being on lots of seizure medications.
I hope you can get your dads constipation issue under control so he won't have any more seizures.