Over voltage - as the engine revs faster so the machine's alternator produces a higher voltage - a Zenner Diode (at least that's how it used to be) sets the maximum voltage - unless the Zenner Diode is broken. At some point the higher voltage will wreck the bulbs.
However it's arranged on your machine it's clear that the voltage regulator is not working and the excessive voltage is causing the bulbs to pop - that's all that can make the bulbs pop.
It will probably be in a small metal block with cooling fins.
Good luck..., January 2015
