Married Men wearing a tallis over their head

What is the source or reason for married men to wear their tallis over their head during davenning. I have observed some people doing this for the duration of davenning and others just for certain parts ie. from Yishtabach or Shemoneh Esrei. Is there a correct practice? Why is this particularly done by married men, as opposed to unmarried men who wear a tallis before they are married? Finally, if one is the shaliach tzibbur for mincha and the minhag of the shul is to wear a tallis, should one wear it over ones head?

The Shulchan Aruch (אורח חיים סי' ח:ב) says that it is proper that ones head be covered with the Talit during the Tefila. The Mishna Brura explains that the reason for this practice is that it "subdues man's heart and induces him to fear of G-d.". However, the Magen Avraham, based on the story of Rav Hamnuna in the Gemara (קידושין דף כט:) says that this practice is only for married men and if someone who is single places the Talit over his head it is considered "Yohara"= conceit. The Sfardi poskin such as the Kaf Hachaim do not mention any reservation for non-married men and Rav Ovadia Yoseph shlit"a. (Yalkut Yoseph on Tzizit , Chap.8.) clearly permits it. Many poskim say that basically one's head should be covered throughout the entire Tefilla. However, Rav Ovadia Yoseph (ibid) says that though it is proper to place the Talit over the head, one may leave the Talit on his shoulders. In the commentary in Yalkut Yoseph, one of the explanations offered as to why people don't keep their head covered throughout the entire Tefilla is because of the distraction caused by the Talit constantly falling back off the head requiring to return it. In regard to the shaliach tzibbur, if indeed the minhag of the shul is to wear a tallit and this is not just the desire of a few, than that is what should be done.