Mormon indicated that these events occurred “in the ending of the thirty and fourth year” (3 Nephi 10:18). Since the sign of Christ’s death occurred in the “thirty and fourth year, in the first month, on the fourth day of the month” (3 Nephi 8:5), these people are gathering almost a year after the beginning of the destruction. This allowed them time to recover from the destruction and to have some semblance of normality return.
With sufficient time to be physically ready, they could turn to spiritual questions, and so they are gathered around the temple. Although the temple grounds might have been a normal gathering area for a large crowd, there may have been some religious festival that had them gathered at that location at that time. There were fall harvest festivals that were part of their Jewish heritage; similar harvest-related ceremonies are not uncommon among agricultural peoples in the ancient world.
If they are assembled for a religiously themed harvest, it may be that there were natural associations with growing things and the death and resurrection of humankind. The seemingly dead seed is placed into the ground, and from that “death” arises new life. That context would easily explain how their minds would naturally turn to the atoning Messiah who had come. It would have been the most natural of conversations.