Having been on the pointy end of the DDoS stick a couple of times I can tell you that it's not necessarily somebody that has something against YC.
I once tracked down a guy that DDoS one of my sites. Turned out he was only testing his new botnet and needed a target. The DDoS took down the whole ISP of my hosting provider, and it caused my them to ask me to find another place to host my stuff.
With 10 Gbps DDoS costing less than a dollar a minute it does make sense to protect one self as soon as you reach the point where somebody would find it funny to take down your site.
