As someone who used to live in Somerville until a few years ago and left partially because traffic was complete ass (but mostly because I couldnt afford to buy there), my theory:

1. Somerville became the 'it' place to live. It's cool, it's fun, it's hip, etc.
2. A bunch of wealthier people with cars displaced poorer people without cars.
3. Now the two way streets which can barely fit one car down them have people parked absolutely everywhere with no room to get by.
4. The rising property taxes and influx of yuppies meant more money to fix things. Yay! Let's reconstruct Union Square, and god knows what else, so more people are forced down a few one way horse-paths.
5. The Redline continues to be a dumpster fire, so even if you want to take public transit if you need to be on time for work, better drive!
6. Now that Somerville is super progressive, they are emphasizing things like making the city more pedestrian/bike friendly. Which is not a bad thing, but means 'traffic calming' measures like: don't paint lines anywhere, let's put curb extensions that jut out into the middle of the street. And oh yes, more bike lanes. Right where those curb extensions are *. So it forces the cyclist right into traffic. And there's already no room to pass anyone on those horse-paths.

TLDR: Got too popular for it's own good. Thank you for coming to my TED talk.

* Curb can extensions can block or NOT block bike lanes. I'm not saying don't build bike lanes, perhaps that wasn't clear. I'm saying putting a curb extension that blocks a bike lane and forces cyclists to veer into the middle of traffic is fucking stupid, and not how it's done in other cities.