Two main things:

* Bad estimates. When people are too aggressive with their estimates and miss them, it derails everyone that was depending on that team's stuff being done at a particular time. It causes blockages that cost way more than an over-estimate would have reserved as buffer time.

* Getting distracted. Dev should work really closely with design and PDM to get things done, but if Design / PDM has already moved on to the next project, it's distracting for everyone when they have to get pulled back in. Then, you have multitasking, and blockages, and stuff doesn't get done as smoothly as it should.