Uh-oh... DS bit one of the kids in his daycare class today

Apparently, he wanted to ride on the bike she was on and became frustrated and bit her!!

The teachers did not expect it because he is so sweet and gentle! He has never bitten anyone ever before! I felt terrible for the other child (they didn't want to disclose who it was but they said she was ok).

What could have caused this and how can I help prevent it from happening again?!