From having flown a lot of free flight there's no natural flare. They just keep gliding until they kiss "the big green thing".

However in your case if you set up the landing approach so it was coming in with either fixed trim or you are sure you held the sticks with no travel then I suspect there's some freak case of the wing wash reacting with the ground and lifting the tail. I have no idea why but if you're sure that there was no possibility of a "stick thermal" or "stick downdraft" then it has to be some odd effect of this one design.

Mind you, given the results of this demo I'm not sure you're game for more testing....