Dear nomesorah
The phenomenon of writing anonymously is quite interesting and amusing, if only because it totally confounds the historians. Who wrote a specific anonymous piece? LOL.
And yet, there were periods where almost all wrote anonymously, especially early kabbalisitic works from the 4th to 9th centuries after the churban. Apparently, the extreme modesty of these authors enabled them to forego the fame. Since these esoteric works were written for the elite few and were not meant to be seen by the hamon am, it’s not surprising that the author kept silent.
As far as halachic works go, there were few, and not anonymous, even though our sources are somewhat sketchy.. The ones we know about are the B’hag written in Iraq (?), the Seder Amram Gaon, and the Machzor Vitri, written by R’ Simcha of Vitri. Obviously the latter two are not anonymous.
As far as perushim of the Talmud, the major question is whether they were written by one person (Rabeinu Hananel ?) or by groups of students of a great teacher, such as Rabeinu Gershom. Most think thet Perush rabeinu Gersom was written by his students and not by him, albeit they were his teachings. As is well known, Rashi was the greatest mefaresh and was able to accomplish what no one since has been able to. Whether he himself wrote all of his perushim or guided and edited what his students wrote in his name is debatable, but his name traveled far and wide very quickly. His Perush was never anonymous, nor was that of the Rashbam . His kuntrusim were so accepted that almost no one felt the need to compete. The result was the expansion of a different form of perush, the dialectic question-answer style of the baa’lei hatosfot. This style was not new, Rashi also used it, but Rabeinu Tam and the Ri perfected it.
When you become more interested, the two major works on Rashi and Tosfos are Grossman’s ???? ????? ????????, ???? ???? ???????? and Auerbach’s groundbreaking work , ???? ???????. Almost all later works are based on these.
Enjoy.