What are "Moseroseinu"?
Rashi: They are straps used to tie the yoke.
Radak: They wanted to snap the ropes of Yisrael and the ties they made, and they agreed to make David king. Because they delayed seven years [from Sha'ul's death], and only now all of Yisrael agreed to anoint him, the Pelishtim wanted to snap the ropes.
Why does it mention casting down their Avosos?
Radak: The matter is repeated in different words; the meaning is the same. Moseros and Avosim are both strong ropes. They are a metaphor for a strong counsel and agreement.
Malbim: Avosim are stronger than Moseros. They are Moseros woven together; it is impossible to snap them. It mentions snapping weak Moseros, and casting down Avosim from their necks. This is both regarding Mitzvos without a [revealed] reason and they can challenge them and nullify them, and rational Mitzvos with a strong reason; they cast them after their pride. Also regarding servitude, they cast off His yoke, and matters about which they swore and were obligated via a Bris and Shevu'ah, they rebelled and annulled the Bris 1 .
Here we discuss Nochrim. Where do we find that Nochrim are obligated due to a Bris and Shevu'ah? (PF)
