I don't think the solution is just to switch sides. My mare has been trailered several times each month for 7.5 years on the drivers side. She has only wound up on the passengers side twice in all of those years. I don't think she is going under the divider simply because she prefers the other side. She has been trailered on the passengers side when hauling with another (larger) horse, but generally seems to prefer the drivers side.
Glad to know that I'm not alone Bogie! I had hoped after the incident 7 years ago that it wouldn't happen again. My barn owner said that I'm good for another 7 years now.
Then I would make my divider longer so she can't get under it. I had a floor length divider in my trailer when I bought it and had to modify it to just a bar type because my horse couldn't balance like he was used to.
Also, in her 2 episodes was it always after your ride on the trip home?