Volvo Repair: Front Brakes on a 98 volvo v40 c/w ABS, caliper piston, slide pins

Question
A couple of years ago I replaced the discs and pads on the front of my Volvo V40.  I have just started to get a metal on metal sound when I use my brakes.  On checking I have found the passenger side disc to be lightly scratched and 1 pad to have gone down to the metal.  All other pads have plenty of life left in them, as in nearly new.  Now I understand I must replace all 4 but was wondering why only 1 pad has worn down and what I need to do to prevent it happening again.  Thank you.

Answer
Lain, Well, one of the slide pins may be seized or getting stuck causing one side to work harder than the other or there may be a blockage or a caliper piston isnt working properly.  Needs to be checked before you replace rotors and pads again.  Roger