A glitch occurs when part of the programming works unintentionally, it doesn't mean the developers had to purposefully put this in.
With this glitch for example, chances are it's able to occur because Bunny Day rocks are "special" rocks. The game is hard-coded to produce 1 fake rock every single day; no more and no less. The glitch is most likely due to Bunny Day's excess of rocks conflicting with the already placed rock/fake rock code and the side effect in this case would be that they are then flagged by the game to be genuine rocks instead of fake ones because the game thinks "There literally cannot be more than 1 fake rock, thus all other rocks must be real rocks!"
That's why this glitch doesn't occur
all the time, only some of the time. It's essentially a mutation in the "cells" of the game's coding. It has the potential to mutate during these circumstances, but not a guarantee.
...
Happy Bunny Day!