Well.. rarely do you get a more definitive answer than from the developers themselves:
How does the Wall Breaker choose its target?
Steps 1, 2, 3
1. It will look for the nearest building/turret that is trapped (fully or partially) by walls
2. It will choose the shortest path towards that building and blow up walls that are in its way
3. After the wall is blown up, it will choose the next building that is still trapped by walls, run straight towards it, and blow up walls that are in its way
So Wall Breakers don’t target walls anymore?!?!!1!!!!?
No. Instead of targeting walls, they aim for the main prize, a building that is trapped by walls. The wall just happens to be in its way, and has to be blown up.
That building is the reason why you need that wall taken down, right?
Do Wall Breakers attack buildings?
No. They aim for the building and blow up the wall that is in their path.
Do Wall Breakers favorite junctions?
No. The Wall Breaker will run in a straight line towards the nearest building. When it encounters a wall, it blows it up.
Can I fool Wall Breakers by using spikes or short decoy wall sections?
No. They won't be fooled by those.
The two main takeaways from this that are interesting:
1. You can no longer lure Wall Breakers with "vestigial" walls.
2. Wall Breakers target the nearest enclosed building. Some have found that you can confuse them near the edge of your main village by enclosing something like a builder hut or tesla. ¡Viva the AI abuse!
As added info, I've found this video from Flammy extremely informative. He shows the Wall Breaker aggro in action, and even has some crude measurements of aggro distance.