>http://en.wikipedia.org/wiki/5_Whys

Looks like 3-year-old me was on to something when I was bugging my parents about how absolutely everything worked.

That said, while this seems like an excellent tool for something like car diagnostics, as described in the wikipedia article, I'd be more hesitant to try it with a person and their motivations unless it was someone I was incredibly close with. I'd especially feel weird doing it with a direct superior. It seems like in most circumstances it would be difficult to word a line of questioning like this in a way that didn't seem like criticizing their judgement.