This silence is apparently a memorial to
the saints who have lived their lives for
God while on this earth.

Revelation 8:3-6
"And another angel came and stood at the
altar, having a golden censer; and there
was given unto him much incense, that
he should offer it with the prayers of all
saints upon the golden altar which was
before the throne.
And the smoke of the incense, which came
with the prayers of the saints, ascended up
before God out of the angel's hand.
And the angel took the censer, and filled it
with fire of the altar, and cast it into the
earth: and there were voices, and thunder-
ings, and lightnings, and an earthquake.
And the seven angels which had the seven
trumpets prepared themselves to sound."