Fuzzy Dwarf Lionfish seizure?

I got a new Fuzzy Dwarf Lionfish last Friday. It exhibited what it seemed would be called typical behavior for a Lionfish new to its environment. It hid under rocks for a day or two. Then it began to show itself more hanging out on rocks and tank n corners in plane site. Its color looked a bit washed out at first but gradually came back. It hadn't really been eating for me though. Although I heard this was also typical. Yesterday I returned to the LFS where I purchased it to see what they had been feeding it. They said they fed frozen Mysis Shrimp. Which is what I had been putting in. Tonight I looked in the tank and saw the Lionfish. Its color was especially striking. It was very dark and rich browns and oranges, like a Monarch Butterfly. No sooner did I comment to my wife about this when it all of the sudden went crazy! It swam around the back of my tank faster than I had ever seen it move. It continued at that speed back to the front, reached the middle front of the tank, swam straight up then straight back down with its mouth open Kamikaze style straight into the sand bed. It continued to force its open mouth into the sand bed for a few seconds before it died, its body standing straight up as stiff as a board. What could have caused this?