Help understanding "rebase"

Here is the situation: An old topic branch containing 3 commits. A dev branch that has recently been merged. To catch up the topic's work before adding it to dev, I expected that rebase would do what I ended up doing manually, detailed below.

Instead, it crunched away for a long time and gave errors applying patches.

So I did it manually by checking out dev, then cherry-picking each of the three commits. 
Actually, this left it on top of dev, but suppose I had created a new branch at dev, 
cherry-picked the stuff from the old topic branch, and then deleted the old topic branch. 
  Now I have a new topic branch with the rebased changes, albeit with a different branch 
name.  Point is, there were no conflicts and the changes were simple, so cherry-picking 
each node was clean.

So, what did the rebase command try to do? I think it may have something to do with finding a common root between the topic and dev, which, due to the merge, was a long way back. Something like this:

	  o--o--   ...  --o
	 /                 \
	A--...--B--   ... --C--D <== dev
	         \
                   q--r--s  <== topic

I was able to cherry-pick q,r,s on top of D without any issues. So why did rebase get in such a tizzy?

--John