When you add hops before the boil the hops tend to coat the surface of the wort and this can cause a lot of tension to build under the "hop blanket" and it can almost explode if you don't keep an eye on it. I have a 14 gallon kettle and I fill it up almost to the top. I stand there while it is trying to boil and use my spoon to break up the surface tension, and I also turn down the heat right when the first sign of surface aggravation appears. Very rarely do I ever have much of a boil over - sometimes a little foam manages to spill over the lip.
Now, if I am not watching it I will certainly have a boil over and doubly so with FWH beers, which would be bad because no telling how much hops you will lose.