yekke2:
That is one possible explanation. Another is that Grindelwald was unable to kill his old friend, and that meant either that the fight was drawn out long enough for Dumbledore to cature him, or that that Grindelwald’s lack of conviction negated the power of the wand.
And, on a more general matter, it is interesting how Grindelwald gained mastery of the Elder Wand simply by stealing it.