1. The sheer volume of commenters and responses took down the system. They cover it by saying a DDoS attack was the cause. 2. They are intent on crippling Net Neutrality anyway, and either designed the system to insufficiently handle traffic and call it a DDos or even DDos themselves. 3. Since there are groups with vested interest in removing net neutrality, and they have deep pockets, I can easily see a few groups moving to protect their interests. In other words, a legitimate DDoS attack.
All 3 seem equally likely to me, especially considering Ajit Pai's stance on NN. This is not encouraging as a citizen of the US.