There are many reasons for this common (yet absurdly nauseating) occurrence. Generally, however, it is basically due to the loose wire which enable it to often be set by pushing the wire in the direction of the earpiece and, if needs be, fixing it into place with a little electrical tape, super glue, or some other bonding agent.

Now and again, if the earpieces have an inline volume turn, that can be the cause. That one is a bit trickier to fix, but you can always try the ‘wire trick’ pointed out above and see if it works. If not, then open the volume wheel and re-solder the cables into place (be warned, this will invalidate most warranties, so if the ‘phones are still covered, just send ‘em back and find replacements). 

Avoiding stuff like this occouring in the future, it’s highly recommended to wrap your cables vigilantly and avoiding stressing your headphones. No, I don’t mean that you should give Motorhead a rest and just play floaty, soothing New-Agey music on them, I mean you shouldn’t have them within your back pocket if you sit down and you must detach them warily from your ears after use (this will sound obvious, but you’d be amazed how many of us just rip ‘em out).

Another thing to take care for may be the jack, if ever the cable is fraying/wearing around the jack, then that may also be a problem. Luckily, like so many things in life, a small amount of electrical tape can really come in useful, just be sure that the copper wiring is tightly bound and it should return to standard usage in no time.

Sometimes, however, it is simply a sign that the earpieces are knackered and no amount of clever tinkering can repair them. Typically, in these instances, the issue is internal. This particular variant on the trouble also catches headphones of any price range, be they Poundland specials or your year old Sennheiser Eargasm headset. I’m reminded of a Shakespeare quote from Hamlet, something about a king and the guts of an beggar, but I can not be bothered to look the entire thing up right now. You get the point I’m attempting to make though, everything dies ultimately, no matter what it cost you.