This is just an impression, but when I tagged the threads in the categories that (probably) most interest me, art and food and I'm gradually working on pets and garden, plus my own threads (now there was a task) I found that the original a2k forum tags couldn't be superceded, Food and Drink stayed forever as the first tag.
I think something similar may happen with a many times posted tag such as 'politics', that it is primary and hard to get erased even if a whole bunch add other tags (not sure of course, glad to be corrected) - that it stays as the general tag for those kind of subjects once it is used.
Similarly, once in a while I've seen someone's personal id kind of tag on a thread so they can follow their own few tags - and no matter how many more appropriate subject tags I add to jostle that out of, say, fourth place in the line, it stays.
My impression back when I was doing all that beginning tagging was also that whoever started the thread, his or her tags seemed to 'win'. I don't think that stands up as an observation, now, but I remember wondering if there was some weight to the tag being from the poster of the thread. (Not that I'm against that, I might even be for it, just wondering.)
I did do one evil thing for sure. I tagged all the appropriate threads 'western breakfast' out of some kind of rampant whimsy. I could be talked into changing those. On the other hand, some day, somewhere, that may be useful to someone.