The big red warning box of doom

Here's the updated DOOM WARNING that appears on a suspicious message in the new Gmail user interface. But why, I ask, would it appear on this particular message that I received? The message in question fully authenticates, it was sent from a reputable ISP, and it was an email message from my city government. It's an email list that I've opted-in to. 

They use an ESP that uses a group of IP addresses to be shared among all clients, so I assume what happened is that the shared reputation has gotten dinged by some bad sender doing something wrong sometime prior to this send. It sucks, though, because this message isn't actually dangerous and the warning is probably going to freak people out.