When in doubt, blame the radio. Justin Bieber has been charged with assault of a limo driver in his native Canada -- which is not related to the Miami arrest or Egg Gate -- and it turns out the situation was caused by the volume of a radio.

Yes, we're serious. #Srsly.

Sources told TMZ that the Biebs was in the backseat of the limo, being driven around Toronto. He asked the driver to increase the volume of the radio while they cruised. The driver complied with the passenger's request, but apparently, it was not to the Biebs' liking or to his satisfaction. And that's when the fight ensued.

An insider said the argument turned physical when Bieber reached over to the radio to turn up the volume. The driver swatted his hand away, and that's apparently a no-no, since the singer then spewed cuss words and smacked the driver upside the head multiple times.

Da Biebs' lawyer says he is innocent, and another source says it was actually a Bieber associate who hit the driver, not the singer. That source would not dime out the friend or share his identity. Maybe it was Lil Za? Just ask Drake Bell. We're sure he has an opinion. But the fact that the insider would not give up the name makes us think maybe it was Bieber who got physical.