I've not nailed down the exact cause as it is usually noticed well after the fact but a Vasari I'll often see structures at partial build % and a constructor idle somewhere else in the gravity well.

My thoughts on it are a possible side effect of the Vasari Colony ship (makes a spare constructor).  Its possible its just a Sins Plus issue but I believe that I may have seen it in vanilla as well

What I think is happening is that the colony ship spawns the additional constructors which then build away and when they expire they just go poof.  Whatever message that occurs on death isnt getting passed so no other contstructor takes over the task.  I've normally seen this mostly in late game (taking over planets) where I claim the planet and crank out a lot of structures (mostly defense/support).  Possibly not noticed early on as most of the time you are strapped for resources and only can throw up a few structure which is more then enough for the bonus constructors to do their builds.

Last game I had in an isolated gravity well 3 completed extractors, 1 hanger 14%, 1 hanger 2%, 1 hanger/turret 0% and the lone constructor was over by an extractor.  I forget what level the colony ship was, this was my "take over this uncolonized cluster" mini-fleet

Also observed Trade pact providing an undesirable 10% modification (noted in thread already multiple times)

While that's probably a bug, you can tell other constructors to work on it manually using their construct ability on the unfinished structure. Should get around it until they fix it.