In fact, Nitish had conveyed his serious reservations over Modi's PM candidature to both Advani and Gadkari. Some political observers also point out that Advani instigated and encouraged Nitish to openly oppose Modi's candidature so that BJP is compelled to declare Advani as its PM candidate.
All these observations can't be corroborated with solid evidence but the way Advani reacted to Modi's declaration as BJP's PM candidate definitely gave credence to these allegations.
Modi's formal declaration as BJP's PM candidate left Nitish shell-shocked. As was widely anticipated, he walked out of the NDA alliance in protest. It turned out to be the biggest blunder of his political career because there was a tangible Modi wave blowing across his state. Nitish had to pay heavily for his misadventure and his party was decimated in 2014 Lok Sabha elections.
2014 Lok Sabha elections proved to be equally disastrous for Nitish Kumar's bête noire Lalu Prasad. While JD (U) could manage to win just two seats, RJD emerged victorious in 4 seats. For RJD, Rabri Devi's loss to Rajiv Pratap Rudy from Saran and Misa's loss to Ramkripal Yadav from Pataliputra added insult to the injury.