How should I interpret an almost-but-not-quite-alphabetical author list?
How should I interpret this authorship convention?
The first (non-alphabetic) authors are likely the main contributors in order of contribution significance as you guessed. Starting from the alphabetic ordering the authors would be those with lesser contributions (but equal to each other).
What fields is it used in, and to what purpose?
I do not know what fields this is specific to, but I would not be surprised if this is a common strategy for papers with many authors irrespective of the field. Ordering a long list of authors with equivalent contributions in some manner simply seems sensible and alphabetic ordering is a simple and common ordering scheme. Unless the publisher has a policy on how this should be handled, alphabetic ordering seems to be a simple go-to strategy.
How should I interpret this authorship convention?
The non-alphabetic people are primary contributors, with their own ordering, while the alphabetic ones all worked together to contribute something that lead to the paper. It might be impractical or undesired to rank authors within the latter group.
You can't assume too much more than this. For example, the last non-alphabetical author need not have been a funder, but could simply have been the least important of the contributors who actually input text into the paper. Also, it's entirely possibly for there to be multiple alphabetic sections, ordered relative to each other based on which contributed component was most pertinent.
What fields is it used in, and to what purpose?
Observational astronomy, for one. I know firsthand since I have just such a paper. The author order is this:
- Primary contributors. Those who did the primary analysis and wrote the paper, in order of who did the most work.
- The leaders of the subgroup of the larger collaboration directly involved in this branch of science (here the Type Ia supernova people in a group looking at all transient phenomena in the sky). The order here is also non-alphabetical, and somewhat pre-arranged in the collaboration.
- The builders of the collaboration, in alphabetical order. Those responsible for setting up the instruments and data pipeline and such.
- The data reducers, in alphabetical order. Those who manually slogged through the raw data at the level of instrumental considerations, getting it into shape so that later reductions at the level of astrophysical interpretation could be done.
I'm sure there are other fields where this also applies. I would expect to see it wherever there is a large group behind the scenes (so ordering based on anything other than alphabetical becomes impractical), but where there is a small group writing each focused paper.