my experience with seizures are pre death seizures which are different than you described. These seizures the whole body vibrates and seizes and usualllly they open their mouths wide their head doesn't violently shake back n forth nor do they release any urine or poop.
Its hard to describe unless you feeeel it yourself the only time I've seen a glider violently shake their head back n forth is when I've given them medicine that tastes bad.
Now that is NOT to say it wasn't a seizure, there are different kinds of seizures and different symptoms and triggers.
The going limp and laying there for a few minutes i've seen once, then he got up and moved around again for a few minutes before seizing again and passing. I was on a video call with a friend of mine when he did it we were both stunned, we both have seen to many gliders seizing right before passing and neither of us saw one do that before after seizing, usually once back to back seizing starts they just don't get up like that.
There are some gliders that have environmental seizures from over stimulation but I don't have any personal experience with that, not sure if any of this helps....