As for how I myself have ended up back at the seminary after being ordained a priest for two years:
My undergraduate work took place at Thomas Aquinas College in CA, where I received a heavy background in philosophy in theology. When I applied to the seminary in 2006, the Vatican had just released a new set of guidelines requiring an extra year of philosophical education, bumping the normal seminary education track from 5 years to 6. My philosophy credits from college more than satisfied the requirements and so I was allowed to modify my course of studies to start on theological work right away.
This meant that I had worked my way through the theological curriculum in about 4 years, leaving me with extra time on my hands.
Fortunately, the seminary at which I was studying had, in addition to the theological faculty, what is known as a "pontifical" faculty––which doesn't refer to a separate group of instructors, but simply that the university has an accreditation to grant not just American degrees but Roman ones as well.
The American system grants seminarians a "Master of Divinity" degree (abbreviated M.Div); the PhD equivalent in this system is a Doctor of Divinity (D.D.). The American seminary system, for obvious reasons, is largely Protestant. The Pontifical faculty, based in the Roman university system, grants a different set of degrees: the Baccalaureate, License, or Doctor in Sacred Theology (STB, STL, and STD). After completing the requirements for my M.Div., I began chipping away at a degree in the Roman system--taking a few extra exams, writing a few extra papers, and eventually receiving my S.T.B. and completing almost all my classwork for the S.T.L. So, I graduated and was ordained with an M.Div and an STB degree––one American and one Roman degree.
The S.T.L., or Licenciate is somewhere between a Master's degree and a PhD in the American university system. In addition to 2 years of classwork, one must demonstrate competency in Latin and at least one modern theological language (such as German, Italian, or French), serve as a teaching assistant, write a 60-80 page thesis, and take a comprehensive exam that demonstrates familiarity with the overall history of theology in general and a few select theologians in greater depth, in the areas of the doctrine of God, Christian anthropology, theological method, and sacraments. If you're really interested in the requirements, you can read about them here.
I really thought I had a shot to finish my STL while still in seminary, but I was wearing myself out getting the work done, preparing for ordination, etc. Finally, I let go of the idea not long before getting ordained. I figured if the Lord wanted me to get the degree, he'd open a door, and let it at that.
Last summer, Father Brian and I were chatting and I mentioned that I had always had an interest in study. He asked why I had never finished the STL, and I informed him that I had very little left in order to complete the degree. He was quite surprised at this, and immediately suggested I look into a summer study program to complete it. (I took this as a divine sign and an answer to my previous prayer for God to open a door. The thought of Father Brian suggesting I leave the parish for 6 weeks two summers in a row to read some pretentious books would have been unthinkable by any stretch of the imagination.) He spoke with the Archbishop, I checked with the registrar in the seminary, and two weeks later I had my academic schedule.
Of course, this was all before there was any talk of Father Brian being transferred to a new parish. My guess is that right now he is sincerely regretting his generous suggestion, being the only priest on hand to minister to the needs of the parish while trying to pack up and move! Let's just say his life isn't neatly packaged into rubbermaid filing boxes; from what I can tell, for the most part it's crammed in large piles into the trunk of his car.
The plan for me, then, is to finish two classes this summer, and then return next summer for a final class and thesis preparation. Hopefully I'll get a lot of the reading done for that during the year, as well; then, of course, at some point I'll need to take my comprehensive exams, which is a one hour oral interview with three professors. That may sound intimidating, but it's nothing compared to being cornered by a parishioner after Mass who didn't like my homily! If I can get through that, there's nothing to worry about ....