The real question here is what is so special about the visual input that it causes a seizure?

Well, many people get headaches. It's not a stretch to say whatever causes a headache in a sensitive person might cause worse.

Edit: to whoever downvotes this, I had seizures when I was 20 years old. Despite numerous examinations including sleep deprived EKG, CT, MRI they never found out what causes them. So if you are downvoting this because I sound like we do not know what causes seizures then you are either ignorant (because we indeed do not know and I am the damned walking example) or have knowledge I really badly need. If the latter then please share links to new neuro research which will help me and my neurologist to decide whether I need to stay on medication or not. I would be glad to stop after two decades.

Here's my speculation on why you got downvotes, based on how I would criticize your post. You're missing a third possibility: That it is a stretch to say that the "worse [than headaches]" could be seizures. It's a somewhat extraordinary claim, and you provided 2nd-hand, circumstantial anecdotes in support of it.