When a package replaces another one the reason the original package was installed might change, i assume depending on whether the specific update pulls in packages that depend on it.
I noticed this with the util-linux-ng -> util-linux transition.
Even though util-linux-ng was explicitly installed during the initial installation of this system, util-linux is now installed as a dependency.
Here's the relevant part of the pacman log:

[2011-03-02 15:24] Running 'pacman -Syu'
[2011-03-02 15:24] synchronizing package lists
[2011-03-02 15:24] starting full system upgrade
[2011-03-02 15:27] removed util-linux-ng (2.18-4)
[2011-03-02 15:27] installed util-linux (2.19-4)
[2011-03-02 15:27] upgraded mkinitcpio (0.6.8-1 -> 0.6.8-2)
<mkinitcpio stuff>
[2011-03-02 15:30] upgraded kernel26 (2.6.37.1-1 -> 2.6.37.2-1)
[2011-03-02 15:30] upgraded kernel26-docs (2.6.37.1-1 -> 2.6.37.2-1)
[2011-03-02 15:30] upgraded ppl (0.11-1 -> 0.11.2-1)

I realise the log isnt that helpful when it comes to proving my point, but i am 100% sure util-linux-ng had been explicitly installed but util-linux is now installed as a dependency.
& i cant think of anything else that might have caused this change.