I've got a ~2 year old microwave that has shown no previous signs of
trouble and is to all appearances in good working order. During
normal operation (melting butter on low power) this morning we heard a
familiar 60Hz buzz, and then sure enough it shot an arc out through
the door to metal rack across a gap of some two inches. Following the
burn marks back, the arc seems to have originated (or at least exited)
beneath the chamber, where the inside of the door meets the body of
the oven and roughly halfway across from the hinges. First the big
question: whatever the failure was, shouldn't there have been a
better path to ground available? Do I have some kind of grounding
issue that I need to fix in before I repair this thing and start using
it again? As for the cause of the arc, since it was able to arc
across such a large gap I assume the failure involves the high voltage
components, not the transformer or anything else seeing AC line power,
so maybe the diode? The capacitor would fail short, so that can't be
it, right? The local repair shop swore this was impossible, and I
haven't been able to turn up any previous posts covering external
arcs, except one from back in 2000 out the top of the oven, so
hopefully this isn't redundant. Has anyone else encountered something
like this or have an idea as to what might be going on? Thanks.

-Chris