Why is a priest serving as a deacon?
When a man receives Holy Orders, he is permanently marked to be a member of that order. So, a bishop is just as much a priest as a priest and likewise a priest is just as much a deacon as a deacon. Therefore, in the Extraordinary Form, a priest or deacon may serve as a Deacon or Subdeacon. Practically speaking, this practice came about because deacons were rarer in the Roman Rite. For hundreds of years prior to the Second Vatican Council, only men who were going to be priests were ordained deacons. (These are known today as transitional deacons.) It was not until Pope Paul VI's motu proprio Sacrum Diaconus Ordinemin 1967 that the permanent diaconate was reestablished. Likewise, Pope Paul VI suppressed all the minors orders. So, there are no longer any men ordained to the subdiaconate outside those societies and fraternities dedicated to maintaining those traditions of the Church. However, even a layman may serve as a Subdeacon except he would not be allowed to wear a maniple or purify the priest's fingers and the sacred vessels at the end of Holy Mass.