Sometimes I think these really big acquisitions are based on the "Politician's syllogism":

1. We must do something

2. This is something

3. Therefore, we must do this

http://en.wikipedia.org/wiki/Politician%27s_syllogism

I wouldn't limit it to the really big acquisitions. If the market says jump, the desperate company does it ASAP and invariably bumps their head on a sharp pointy object.