I've never had a failure I could attribute to zinc/tin whiskers, though my sample set isn't enormous and I haven't ever really had a rash of power supply failures like you describe that would make me go on a hunt for a root cause.
I'd be looking at more conventional problems (bad capacitors in the power supply or a transient electrical fault rate pretty high, especially since you say you had two "large occurrences" of PSU failures), though it sounds like you already have.
My short list in case it differs from yours/your electrician's:
Electrically: poorly stabilized power because of a wonky UPS or PDU/CDU, ground faults, etc. If your "large occurrences" were in areas served by the same power distribution equipment this becomes more likely.
Environmentally: temperature & humidity; Check inlet/outlet temperature of your equipment (especially if the failures happen in the same physical area of the datacenter: You may discover an airflow/cooling issue causing your gear to run hot).
Equipment/Manufacturer QC: check the dead power supplies for bulging/blown capacitors, especially if the failures are in units bought around the same time. Make sure you're not pushing the power supplies too hard (lots of hard drives & power-hungry CPUs may warrant a bigger PSU)