Why did he say "Achar Avi Es ha'Aretz"?
Rashi: He muddled up their minds and salvation, like Mayim Achurim (cloudy water).
Alshich: "Es ha'Aretz" excludes me. I am not liable, for I was Shogeg. Also, I had only Devash, and not bread!
Malbim: He decreed improperly.
Was Yonason trying to annul Sha'ul's Shevu'ah? This is improper, due to Kivud Av, and honor of the king!
Rishon l'Tziyon (of the Ohr ha'Chayim, 27): The Rambam writes (a) If an oath is imposed on Ploni, he is obligated only if he answered Amen. Yonason had not heard the Shevu'ah, i.e. he had not answered Amen to accept it. (b) If one swore, and revealed the reason, or it is clear from the context, and the reason is found to be mistaken, the Shevu'ah is invalid; it need not be permitted. Also, we do not decree on the Tzibur something that most people cannot fulfill. Yonason himself debated about this. He did not explicitly permit others, just he said that one who wants to rely on his Heter, he may rely on it. The nation heard, and [relied on it -] "va'Yochal" (verse 32).
