There are numerous causes for this everyday (yet absurdly frustrating) occurrence. Generally, however, it’s simply due to the loose wire and can frequently be set by pushing the wire towards the radio earpiece and, if needs be, affixing it into position with a small electrical tape, super glue, or some other adhesive.
Occasionally, if the headphones have an inline volume turn, that may be the cause. That one is just a little difficult to mend, but you could possibly always try the ‘wire trick’ described above and see if it works. If not, then open the volume controls and re-solder the wires into place (be warned, this can invalidate most warranties, so if the ‘phones are still covered, just send ‘em back and get replacements).
To avoid stuff like this occouring in the future, it is recommended to wrap your cables cautiously and avoiding stressing your headphones. No, I do not mean that you should give Motorhead a break and only play floaty, soothing New-Agey music on them, I mean that you shouldn’t have them inside your back pocket if you sit down and you need to detach them cautiously out of your ears after use (this may sound obvious, and you’d be astounded how many individuals just rip ‘em out).
Another thing to take care for is the jack, if the cable is fraying/wearing around the jack, then that may also be a problem. Luckily, like a lot of everything in life, a small amount of electrical strip can really come in useful, just ensure that all of the copper wiring is tightly bound and it should return to standard usage in no time.
Now and then, however, it is simply a sign the earpieces are knackered and no amount of clever tinkering can fix them. Generally, in these cases, the problem is internal. This unique variant on the problem also attacks 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 plus the guts of a beggar, but I can’t be bothered to look the entire thing up right now. You get the purpose I’m attempting to make though, everything dies eventually, regardless of how much it cost you.