>>44209775It was multiple factors, actually.
Ash becomes unnerved that Sawyer, this rookie trainer, is getting the better of him. Because Sawyer actually trains his team instead of fucking around like Ash does. While Ash went to Serena's showcases, fought Team Flare, and helped people and Pokemon overcome their problems, Sawyer was the one taking his training seriously. He gets FOUR badges in the same timespan that Ash got one (from Fairy to Psychic), and several of his Pokemon evolved. Sawyer gets an additional THREE badges, including the Icicle Badge, while Ash didn't get his last badge. Many of Sawyer's Pokemon got stronger and evolved to their highest levels, and what's more, Sawyer actually developed and executed proper countermeasures against all of Ash's Pokemon. Ash loses, not just because of the stress of Ash-Greninja, but also because he gets unnerved at the thought of this rookie trainer surpassing him. Ash is too lax in Kalos, and that's why he is falling behind. Ash lost to Sawyer because of Ash-Greninja, although Sawyer assumed Ash would have beat him at full power. Sawyer accompanied Ash to his last gym, the gym he just took on, believing Ash could easily win the badge there. He doesn't. Ash is still riled up from losing to Sawyer, and failing to master Ash-Greninja. He loses to Wulfric, with the latter only using 2 Pokemon against Ash's 3 (including Ash-Greninja and Talonflame).
Ash realizes how shitty of a trainer he is, and he screws off to Winding Woods. Sawyer was a threat to Ash in the league because, unlike Ash, he takes every defeat with stride, and he improves himself. Sawyer literally designed his team to be an anti-Ash team, and he would have won were it not for plot armor and some quick thinking on Ash's part.
