R' Yochanan ben Zakai, in leading the Jewish people during and immediately after the Destruction of the Second Temple, issued many decrees aimed at responding to the new post-Temple reality and preserving Judaism. When I see one of these decrees recorded in the Mishna, I find it interesting to think about what particular problem R' Yochanan was trying to solve, what had caused the problem, and how he chose to address it.
In Mishnah Rosh Hashana 4:4, we find the following decree:
אמר רבי יהושע בן קרחה ועוד זאת התקין רבן יוחנן בן זכאי. שאפילו ראש בית דין בכל מקום. שלא יהוא העדים הולכין אלא למקום הוועד
Rabbi Yehoshua ben Korcha said, "This too did Rabban Yochanan ben Zakai ordain: even if the head of the court would be in any [other] place, the witnesses need not go anywhere [else], but to the place of meeting [of the court]."
The issue here is that in the existing process for declaring a new month, the formal declaration was made specifically by the head of the Sanhedrin. R' Yochanan decreed that thenceforth, the process could be completed without the head if he was not available in the Sanhedrin's meeting-place.
Clearly, this decree is related the the fact that after the Destruction, the Sanhedrin was relocated from Jerusalem to Yavneh. Presumably, it has something to do with post-Destruction turmoil and uncertainty.
I'm curious, though, what particular circumstances led R' Yochanan to make this decree. After the Sanhedrin moved, did its head not move with it? Did he spend more time traveling, for some reason? Was R' Yochanan just hedging against general uncertainty about the locations of the Sanhedrin and its head, now that the fixed point of Jerusalem had been destroyed? Did some particular incident spur this decree?
For some other decrees, the Mishna tells us about specific incidents or problems that led to them. (See, e.g. Rosh Hashana 1:6 and 2:2.) Do we have any source that tells us what, specifically, led to this decree?