What is the motivation of horseflies?

I was outside mowing the lawn the other day, and I couldn't help but notice that I was getting absolute dive-bombed by a swarm of horseflies.

The horseflies that we have in our yard bit me once last year on my back, and I seriously had a noticeable huge one-inch diameter bite for about a month.

Now, whatever. I get the motivation for most insects.

Ants wanna build huge mountains and collect stuff, cool. Bumblebees want to collect pollen and suicide kill you if you get anywhere near them, I get it.

But what I don't understand is how I can be minding my own business, literally no where near anything but grass in a 30 yards in any direction, and all of a sudden finding myself to be in a complete war zone.

To me, there are few things more frightening than knowing there's a horsefly hovering around my head. Wasps and bees might sting you, but usually afterward they give you a "TAKE THAT" and fly away.

Not horseflies, they bite you and bite you. And then, just when you think you killed the horsefly that's been biting you, one of his stupid friends comes and starts dive-bombing you.

Horseflies are nature's terrorists.