This is most likely the reason it was done IMO. These idiots do these types of things for attention and guess what? The media will give them all they want. Makes them feel like super villains (Colorado).
I don't believe runners were targeted specifically. I feel it was a because of other factors, most importantly the on site media, the large dense crowd and that today is patriots day (celebrating the first battle of the revolutionary war.).

very sad and disturbing.

True, either way it's just fucked up...