There are many causes for the familiar (yet absurdly frustrating) phenomenon. Generally, however, it is simply due to that loose wire which enable it to regularly be fixed by pushing the wire towards the radio earpiece and, if needs be, affixing it into place with a small electrical tape, super glue, or other glue.
Sometimes, if ever the headphones have an inline volume turn, that may be the source. That one is a bit difficult to mend, but you could possibly always try the ‘wire trick’ pointed out above and see if it works. If not, then open the volume controls and re-solder the cables into place (be warned, this may invalidate most warranties, so if ever the ‘phones continue to be covered, just send ‘em back and find replacements).
To avoid stuff like this occouring in the future, it is highly recommended to wrap your cables vigilantly and to avoid stressing your headphones. No, I don’t mean you should give Motorhead a rest and only play floaty, soothing New-Agey music on them, I mean that you shouldn’t have them in your back pocket when you sit down and it is best to remove them cautiously out of your ears after use (this will likely sound evident, and you’d be stunned how some people simply rip ‘em out).
Another thing to look out for could be the jack, if the cable is fraying/wearing around the jack, then that can also be a problem. Fortunately, like so many things in life, a small amount of electrical strip can really come in useful, just ensure that the copper wiring is tightly bound and it should return to normal usage in no time.
From time to time, however, it’s simply an indication these headphones are knackered and no quantity of skillful tinkering can mend them. Usually, in these instances, the issue is inside. This unique variant on the trouble also attacks earpieces of any price range, be they Poundland specials or your year old Sennheiser Eargasm series. I’m reminded of a Shakespeare quote from Hamlet, something about a king and the guts of a beggar, but I can’t be bothered to look the entire thing up right now. You get the point I’m attempting to make though, everything dies eventually, no matter just how much it cost you.