This is one of the many things that happen during mass that confounds folks. Having been formally catechized in the US but informally catechized in Mexico, this confounded me to no end! I'll explain what I mean by that; but first, the simple answer.
The absolute simplest answer to this is that those assembled are simply doing what the deacon (or priest) is doing (#134). Look carefully, and you'll see the deacon signing his forehead, his lips, and his breast before reading the Gospel. The mass is a community prayer; those assembled aren't watching clergy perform a mass, but are participating with him as he leads the prayer. And just before the Gospel, the deacon (or priest) leads us in signing ourselves: we sign our forehead, our lips, and our heart.
Doing an Internet search for this will yield several pages that say the people sign themselves so that the "word of God be on their mind, in their speaking and in their hearts so as to be faithful witnesses to the gospel." Digging a little deeper, I found the Douay Catechism, from 1649, and it said, "They sign themselves on their foreheads, to signify they are not, nor will be ashamed to profess Christ crucified: on their mouths to signify they will be ready with their mouths, to confess unto salvation: and on their breast to signify that with their hearts they believe unto justice." (pg. 128)
The Douay Catechism excerpt reminded me of another time we get a sign on our foreheads: Ash Wednesday. What struck me is that both times the sign on our forehead is for the same reason: we are not ashamed to proclaim Christ crucified.
So why would any of this confound me "to no end"?
As I mentioned, I received some informal catechesis in Mexico. I was taught to start a rosary by signing my forehead, then my lips, then my heart, and then I do the sign of the cross. And through all this, I say, "Por la señal de la Santa Cruz, De Nuestros enemigos, Líbranos Señor Dios Nuestro, en el nombre del Padre, y del Hijo, y del Espíritu Santo." ("By the sign of the holy cross, deliver us from our enemies, O our God, in the name of the Father, and of the Son, and of the Holy Spirit.").
That catechesis, however, did not mention whether or not this was the same singing that was done during mass before the Gospel. The observer in me sees people during mass sign their forehead, lips, chest, and then do a sign of the cross, making me think they are signing themselves as if they are starting a rosary. And this is what confounded me to no end: saying that prayer immediately before the Gospel seemed out of place. It is, afterall, a prayer of protection, and one doesn't need to be protected from the Gospel!
Thus ends my confoundment on this issue. As for those people who add the sign of the cross after signing their forehead, lips, and breast? Perhaps they're over-signing out of habit. And I'll never complain about people being in the habit of praying the rosary.
