I don't see the "Shake screen" action on your screenshot. Are you sure it's triggered only once? You can add something like Browser Log "Shaking" into the same event, open browser console and see how often this event is triggered.
The shake command was used in the sheet for the level, and is just a "if playerbox collides with X, then shake."
I can stick that sheet up too, if necessary. It's just so basic, I figured the problem might be in my camera controls.
I'll try out your browser log tip, and see what it nets me. Thanks!