Well, that's what the post says. They found a bug in the update. They didn't say there was anything wrong with the servers.
It's pretty common in all areas of software. I regularly take websites down and put a "maintenance" page up, I'm not actually doing maintenance most of the time. That's just the easy way to keep everyone out whilst you do whatever it is you're actually doing, where if you let people in it'd either mess up what you're doing or they'd see errors whilst you fiddled with stuff.
The most likely explanation is that they saw something in the update so bad that they didn't even want to risk having people play with it, unlike the last patch where the bugs weren't gamebreaking and they let people continue playing. As such since some people already applied the patch the only thing they can do is create another patch to fix, and that's not going to be quick. Even as a first party, it has to go through submissions to Sony for safety once it's ready.