As far as I can tell from the article, it would both make sense if they were built before or after the attack. It's also mentioned in the article that they are not sure.
They could have been built before or after the Ashdod rebellion was put down, either at the initiative of the locals or at the orders of the Assyrians.
If it was before the rebellion, it would make sense. They would simply want to make sure they were thoroughly protected for when Sargon II would try to break the rebellion.
Assyrian inscriptions reveal that at the end of the century, Yamani, the rebel king of Ashdod, led a rebellion against Sargon II, the king of the Assyrian Empire.
If it was built after, it would also make sense. It would make the story less interesting though (maybe that's why they mentioned the rebellion: they need good stories to get more funding). There's many reasons why the Assyrians would build fortifications, even if they were the head honchos back then.
1. Protection from other (possible) invaders of marauding tribes.
2. To enhance the status of the city.
3. Walls could be built not only to keep people out, but also used as a control against its inhabitants.
There's more possibilities but these seem the most logical. I don't think 2. would be the case, since the people from the city rebelled (or at least supported Yamani). So it seems unlikely a ruler would reward the population with enrichments to the city. It's more likely a control that the inhabitants would rebel a second time. However, it seems to me that it would be choice 1.. If you look at where Ashdod-Yam (now Ashdod-Sea) is situated, it makes sense to build walls. It's a port town, which means it's an excellent target for pirates and marauders. You can see where it is by looking at modern day Ashdod (Ashdod-Yam is in the South).
Just speculation of course. But perhaps it will give you some ideas as well. If it does, please share
http://en.wikipedia.org/wiki/Ashdod-yam
http://en.wikipedia.org/wiki/Ashdod