Becasue you showed a sudden interest in removing extra metal that would
trigger an alarm on the metal detector, they then waved the wand over you to
ensure you didn't do it to hide the small amount of metal in the firing pin
of your plastic concealed firearm.

Reducing the overall level of metal which would otherwise trigger an alarm
has to trigger a warning with the security guard. Doing it in full view and
probably looking scared of being detected while doing it would arouse extra
suspicion.

I would much rather the guard spend extra time checking you properly after
you had shown a suspiciouos act to hie the amount of metal in total you were
wearing.

Of course, if I had been a security officer I would have held a gun at your
head, kicked you to the ground and made you strip while the search was
carried out as you were obviously suspicious!

Hope this helps,
Peter