Cold, rain, or even local kids playing with not-yet-dry concrete can cause all sorts of issues.
Shortages of steel or wood, or a small accident that breaks something, can often set work back weeks. When I did welding I remember waiting around for for hours for more rebar to show up -- and you bet your ass we billed for that.
On one job some goober fell through an unfinished staircase due to it being mostly (but not totally) completed; it halted work and were down a skilled worker. IIRC dude cracked a few ribs and had shoulder problems, not sure if he got workers comp or disability -- but should have.
One a different gig someone kicked over all of the porta-pottys and turned on all of the outdoor faucets, creating a huge mess of poo and mud. Later on that job site someone stole a lot of copper, and I think at least 2 pallets of cinderblocks, as well as a non-trivial amount of fence.
Meanwhile, I've got an IT job now, and when it's cold and rainy I can sit at home and jam out lines of code as long as my wifi and HVAC mostly works.
Same as with software really.
The external factors are also a thing, material shortages or finding something unplanned that needs fixing (for renovations)