Quinn has been terrorizing top lane on and off for quite some time now.
Other ADCs don't work in top lane for a variety of reasons, the main one is that the they're very susceptible to all-ins, and that's basically what happens in top lane during the laning phase. There's almost always one clear victor when the lane is over. However Quinn has Vault to deal with that while having constant harass. Her kit allows her a strong lane without a support.
The problem does have to do with Quinn specifically. Vayne hasn't worked in top lane since last year, and the problem with Graves is a balance issue that effects him in every role and at all portions of the game.
When the Marksman changes were announced everyone wanted to avoid making Quinn's abusive top lane pick even worse, but it looks like that wasn't avoided with how well she's doing there again.