Hmm. In our case, it was because the team had one person that was mainly responsible for tests (in the beginning). That guy was full of 'what if's'.
It always reminded me a bit of a book I got, one called 'Java Puzzlers'. It had all these kinds of weird things that can go wrong if you program them a certain way. I suspect he read that one, and instead of writing good code, wanted tests to make sure no-one accidentally made a puzzle!