Construction prioritizes floors over other construction jobs.

[Rock5]

I've noticed that when you have a lot of construction to do and then you lay down some floors, everyone pitches in to do the floors and nothing else gets constructed. So it seems that floors are given higher priority than anything else. I would expect floors to have the lowest priority and would be done last after walls, doors, turrets, etc. Or at the very least it should have a equal priority. I've noticed this on a number of occasions, no other construction is done until every single floor is finished.

[Tynan]

It has to do with how they try to finish several blueprints at once, and floors take tiny amounts of materials. There's no special priority for floors.