Whatever the reason is, it is rare and random. I repeated the turn several times and could not reproduce it again.
Baron your explanation sounds reasonable to me. The game where this happend is quite advanced and stellar manipulation is very frequent: one AI empire blew up a star in a system on the other side of the galaxy in the same turn and planets are created like rabbits.
I just accept this as an undocumented "feature": The planet was teleported by some time vortex to a distant future.
