What will make them stumble?
Rashi: What they wanted to make Daniel stumble in, it will be reversed onto them.
Radak: Their tongue, i.e. the evil words that they spoke about me, will return on them - "va'Yipol b'Shachas Yif'al" (7:16).
Malbim: Their tongue caused them to go on the traps. They were trapped through what they set to trap David. E.g. David told Do'eg "Ficha Anah Vecha" (Shmuel II, 1:16). Initially he said that David wants to kill Sha'ul; he stumbled in this, and said that he killed Sha'ul. He was killed for this!
What is "Yisnodedu"?
Rashi: They will shake the head and laugh at them.
Radak: [All who see their turmoil and downfall] will be shaken and fearful of Hashem.
Malbim: All who see them will be shaken and say that their tongue made them stumble.
