I'm pretty good with seasickness, but I have been seasick many times, I sailed commercially for 20 years and I have observed all kinds of factors contribute. The worst factor for me is fatigue. If I've been up for a couple or 3 days with little to no sleep fighting adverse weather, I find seasickness eventually will work it's way just like mosquitoes, given time will find a crack to get into.
I have also observed pregnant women are highly susceptible, older people can be more susceptible. Hung over people are easy targets. Fear and uncertainty can be major contributing factors.
Sea state is a big factor too. The conditions you describe sound pretty bad for a small boat. Maybe age was one of several factors?
My worst seasickness was a long Search and Rescue operation I participated in. We were a small crew on an aluminum vessel on Lake Huron searching for a 12 year old boy who was blown offshore in a 12' boat with only a swim suit on.
I was very tired, I was cold, I definitely felt some anxiety about the boys welfare. The sea state was not bad, maybe a meter, but you couldn't see them coming in the dark. Also I was using night vision goggles, which always give me a headache.
On the second night of this, I felt so rotten I asked the permission of the Captain to go below for a nap, which he agreed to. I slept for a few hours and felt much much better.
The boy was found safe and sound, although very cold and uncomfortable.
I think it's good you are analyzing causation, it will help you to combat it in the future.
Another bad one was on a chemical tanker in the northern Atlantic, battling what would have been a survival storm for a yacht. Our ship was Hove too and taking it very well, I had ample sleep. It lasted a few days, again, I think a big factor was anxiety. Even Hove too big waves were crashing over the focsal, sweeping the full length of about 300 ft of deck and slamming into the base of the accommodations with rather frightening effect. In that storm I think the only major factor was fear, which in hind sight was probably unwarranted, but felt very real at the time.