Overvoltage of the front end regulator is a credible theory
but in failure analysis you should beware of your impulse
to believe the first good idea that comes along. Rather,
collect the possibilities and work the evidence to get the
probabilities, as priorities for deeper digging (like, there
may be other ways to fry a switching regulator, but some
may not be credible given the assembly and access and
local threat "population" - maybe a "screwdriver short"
on the output, while a credible way to damage a part,
is not credible in the hardware as it sat).
There is always some "wandering around in the basement,
in the dark, looking for the light switch" element to FA in
the early going. Don't try to run ahead, take your time
and diligently explore the failure modes, effects so your
analysis has a solid chance of including the real villain.