I think the idea comes from the practice of having “torch bearers” positioned in front of the altar during the Eucharistic Prayer. My understanding is that there is nothing in the official liturgical books that directs this.
But Peter Elliott (who is now a bishop) wrote in Ceremonies of the Modern Roman Rite, Chapter 6 “Solemn Mass”:
“After the incensation of the people, the thurifer waits at the center facing the altar. The torch bearers come from their places in or near the sanctuary and line up with the thurifer. Together they make the customary reverence. Led by the thurifer, they go to the sacristy or some other place near the sanctuary and light their torches. They wait there until just before the Sanctus.” (Peter Elliott, Ceremonies of the Modern Roman Rite, Ignatius Press, 1995, ISBN 0898705266, page 147).
Then on page 148: “During the singing of the Sanctus, the thurifer leads the torch bearers to the sanctuary, walking in pairs and carrying their torches in the outside hand, the other hand resting on the breast as usual. They form up in front of the altar, either across the sanctuary or in two lines facing the altar, or according to the shape of the sanctuary.”
His directions are that these torch bearers be in the sanctuary. But perhaps, in some cases, they will obscure people’s views of the altar. So someone may decide it is better to have them lower down, outside the sanctuary.
My opinion is that having the torch bearers or thurifer in front of the altar is wrong. The Ceremonial of Bishops has (in the description of the Stational Mass of the Diocesan Bishop):
"153 … The deacons stand behind the concelebrants, but in such a way that one of them may assist at the cup and the book as needed. …
“155 … One of the deacons puts incense into the censer and incenses the host and the cup at each elevation.”
So this deacon, doing the job of the thurifer, is directed to be behind the concelebrants. (Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 57).
