The Daily had a good piece about this race. Apparently AIPAC specifically targeted him because he already had that fire alarm thing tarnishing him, and his district had a strong challenger that people were very familiar with.
AIPAC knows money alone can’t win a race. They did what a lot of political action groups do now. They want to use their money effectively, so they specifically picked a race that looked like it could be influenced. Then they threw record breaking amounts of money at it.