You really shouldn't expect the edges of your seat to hold you in, you need to hold yourself in either by bracing fore/aft or triangulation with your footbraces, or most especially highsiding. I don't see really any excessive getting thrown around, it's hard to say whether or not you "should" have stayed in your seat that one time. Getting tossed out of your seat happens, and the best way to avoid it is to highside into the wave. One thing I did notice, the way you hold your handles struck me as odd. I hold on over the end, and it really allows me to move the oar around, I can pivot it on the ball of my hand as a transition to having the oar handle in front of me to behind me. It also really lets me brace against it when I need to. It looks like you are holding on pretty far down the handle, almost hitting the shaft? I could be wrong, video is deceiving sometimes. I use a lowback and prefer it. I like to be able to move around my boat, highside and ride it to keep it down. I have a different frame, which I think is a factor also.