Hi Mike,
without getting to 'geeky' the reason it asked for your password is that all files and folders on a Mac are 'owned' by someone ! (Unix permissions !)
The files that you were deleting were installed/owned by 'Root' (your system admin account), so to delete them your computer asks you if you have the correct permission to delete them...your password !
Sometimes when installing software your Mac also requires your password this again is because the 'Software Installer' wants to either remove/install/add etc files that are or need to be owned by "Root"...
It's just the computer's way of making sure that you 'really want to do that !"
regards
Ric