Within a given phase, and to some degree even across multiple phases, there is an expectation that a significant amount of work within multiple processes are happening at once and not serially.
So, you need to start the process of estimating costs but you know you need to determine your risks (and figure out how much effort will be needed for QA, and figure out your resource plan, and so on and so on ...) before you can nail down the budget so you have to do those, along with many other processes, at the same time.
Thanks for the clarification!
I could notice atleast a couple of inputs that fall under this category.
I need to go through the related processes one more time to understand the impact of non-availability of such inputs.
I'm currently in Florida without my PMBOK Guide (which should never happen to ANYONE who is preparing for the PMP Exam by the way... ALWAYS have it with you.)
Anyway... My immediate answer without looking into the book would be that we need to remember that project management is circular and not linear. We do these processes again and again. So an input from a "later" process into an "earlier" process would happen the 2nd time around.