There probably isn't any tested and validated theories, but are there currently sound ones?
Offering up my own experience as food for thought.
Have gotten 6 guild mark leves so far, 2 Conjurer, 4 Thumaturge, 0 Weaver. I check all three cities every leve cycle for all relevant regional and local leves.
My first two were the conjurer ones. After that I started leveling my Thumaturge a lot more (Conjurer duos with my Gladiator friend, Thumaturge solo farms). I would think it's difficult to be based on play time of job, but given that there is a fatigue system it's not impossible for next cycle's leves to be dependent on which job is 'most fatigued'. Or just random luck. I now wish I didn't level THM so I wouldn't even have to see THM marks. I plan to put my THM in the shelf for a few cycles and see if my Conjurer marks comes back. I don't even want THM marks right now. Just Conjurer =(
Share your experience please, and let's see if there's enough data to at least start forming educated guesses.