in the comic it's not very clear, i know, but in the movie it was because of stress and pressure. Katanas aren't that thick, so they're likely to break at some point (especially if Raph is the one against them )
Very dynamic, although I wish you'd done the surprised look on Leo's face when the swords snap. The way you do the rain is so spot on here, from the amount you use to the way it plays off of the different surfaces. Great job.