The home page questions are cached, but apparently after posting new answer the last activity is updated even in the cache causing weird behavior:
Looks like the question kept its old place, but displaying the real last activity time.
Related bug reports:
I offer a way to make it look better: either show the old last activity time, or apply the sort again so that it will be on top.
As questions also won't appear in the home page for about a minute after asking them, the first option might be more consistent with existing behavior.