Suddenly, my builders have just stopped working on building projects. Any time I click on them, they're idling. All the rest of the laborers are doing their jobs. What's up with these slackers?

Idling means either of two things

1) There are no unassigned tasks for the person to do as a builder, and no general labor ones to help out with either
or 2) The person is tired of work and is taking a break.

Note that a building project has three phases - site clearance, delivering materials and the actual construction. Only the latter is builders' work; the earlier phases are general labor. Builders for whom there is no construction work ready to go turn to road-making before general labor.

One thing I've noticed time and again - if the person to whom a task has been assigned decides first to go home for lunch/go shopping/take a break/etc, the task just has to wait for that person to come back; it won't be reassigned to anybody else, not even to somebody who is idle for lack of work. Unless we find a way to intervene.