In Havdalah, I noticed a virtually universal custom: when the one who is saying Havdalah reaches the Passuk of Layehudim Haytah Orah (ליהודים היתה אורה ושמחה...), those who are being Yotzei Havdalah say the passuk aloud.
(One variation of this custom seems to have those around say it while the "Chazzan" is quite, and then the Chazzan repeats it. The other seems to be the Chazzan saying it together with those around)
Either way, what is the source of the custom? Why is this Passuk said aloud?