Tekkie® wrote, on Fri, 09 Jan 2015 16:19:34 -0500:

That was suction of air in the boot. Get a spark plug boot pliers.
Also look at the part numbers on the old plugs.

Actually, in hind sight, the metal cap on the boot was stuck so
tightly to the spark plug by friction alone, that the boot was
"stretching", which, when I let go, allowed it to "snap back".

The old spark plugs were nearly pristine on the ends, as there
was no corrosion whatsoever on the ends so the boots were clearly
watertight.

I did put some dielectric grease in there anyway, along with
anti-seize sacrificial-anode paste on the spark plug threads.

Power went down twice more since this thread was started, and
everything worked just fine.