No, the nature is not preset and can be any nature. Chances are that either its your luck, or you're accidentally carrying a Synchronize user with Modest Nature.
As for the IVs, Groudon will have any 3 full IVs, which can be anything at all. The first Groudon could have full IVs in HP, Attack and Defense, while the next one could have Special Attack, Special Defense and Speed.
Hope I helped!