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.