- The tool you planned to use has a bug/defect that blocks you.
- The people who said they could give you some information you need, can't.
- You misunderstood or were misled as to the capabilities of a tool you need.
- The parts you need don't actually fit together as planned.
- The documentation you were relying on is wrong.
- A delivery you are relying on won't actually be ready on time.
- A process that's worked every time has an unhandled edge case that you're going to trigger in this project.
- A regression in a software package will block you because you can't use the previous version for other reasons.
The more moving parts there are, the more likely it is that one of the interface points between them will block or delay you in one of these ways, potentially for weeks, maybe even months.
So for any nontrivial project, you should expect a 2x-8x delivery inflation over your "safely padded" estimate. You have a 75% chance that you'll deliver within 2x of your padded estimate if you've made a good estimation, sliding to worse as the number of moving parts increases.