Upside-Down Lemon Squares?
I assume the crust floated and the curd displaced around it, rather than anything actually "flipping." Actual flipping would fall under the "other members of the household" comment.

Floating merely requires the shortbread crust to detach from the bottom of the pan (perhaps aided by being cool) and lemon curd to flow under it, slowly - it may have been doing this (but not broken the surface) before it was refrigerated, and continued in the refrigerator. Nothing needs to be overly fluid for this to happen - flow in thick/viscous  liquids just needs plenty of time.