Tracking Project Slippage

Hey all!  So I've seen 2 posts about tracking slippage for a project.  If we had Wrike Analytics, I can def see how we could easily track how much the project slipped. 

But I'm wondering if anyone has a recommendation for tracking why the project is slipping. So for example, Project A is put on hold because priorities changed and we now need to work on Project B.  How are people tracking that or identifying that so we can see the impact of Project B on A, and maybe not just A, but also X, Y, Z, etc... 

Hope that makes sense. Interested to see how the community is doing this!! Thanks all!