Just a good antivirus scan should do the job for your own PC and a little bit of work with your hosting provider should help to check you hosting server. It not Unity members' computers, trust me. Botnets are comprised of just random computers.

As for reasons to attack... there're plenty. We have seen attacks coinciding with competitor's product launch. No proof, just a quick guess.

Good thing is that it is now a rule that such attacks ALWAYS bring something good with them - new knowledge, see more new technologies, good changes. They work like a good caffeine boost. Even now, they helped us to find a way to slash our bandwidth costs, improve loading speed and server security.