How should I interpret an almost-but-not-quite-alphabetical author list? I just noticed a weird aspect about this paper (which was of interest here and recently came up again here). If you look at the author list, it reads


  L. P. Gaffney, P. A. Butler, M. Scheck, A. B. Hayes, F. Wenander, M. Albers, B. Bastin, C. Bauer, A. Blazhev, S. Bönig, N. Bree, J. Cederkäll, T. Chupp, D. Cline, T. E. Cocolios, T. Davinson, H. De Witte, J. Diriken, T. Grahn, A. Herzan, M. Huyse, D. G. Jenkins, D. T. Joss, N. Kesteloot, J. Konki, M. Kowalczyk, Th. Kröll, E. Kwan, R. Lutter, K. Moschner, P. Napiorkowski, J. Pakarinen, M. Pfeiffer, D. Radeck, P. Reiter, K. Reynders, S. V. Rigby, L. M. Robledo, M. Rudigier, S. Sambi, M. Seidlitz, B. Siebeck, T. Stora, P. Thoele, P. Van Duppen, M. J. Vermeulen, M. von Schmid, D. Voulot, N. Warr, K. Wimmer, K. Wrzosek-Lipska, C. Y. Wu and M. Zielinska


In particular, it splits into an initial, non-alphabetic component, 


  L. P. Gaffney, P. A. Butler, M. Scheck, A. B. Hayes, F. Wenander


and then an alphabetic list from M. Albers through M. Zielinska. 

How should I interpret this authorship convention? What fields is it used in, and to what purpose? My initial reaction would be to assign to the initial component a ranking down in 'importance' more akin to the "first author did most of the work, middle authors supported, and last author sponsored and oversaw the work" convention used in fields with smaller collaborations (at least in physics), but that leaves the alphabetic component in an awkward position, so I'm not sure my interpretation is right.
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.