What is the halacha about these burials? How was this allowed to be done? According to archeological historians after this the bones were placed in "ossuaries" and many have been found. Kol Tuv, Thanks,
I assume by "these burials" you mean the practice of Likut Atzamos - burying someone temporarily and then disinterring the bones after complete decomposition of the flesh. This is a well documented practice, mentioned many times in both Talmuds. Probably the definitive statement of this practice is to be found in a Braisa in the Yerushalmi (Sanhedrin 30a).
Probably the driving force of this practice was that it was considered a great honor for the Niftar to be buried together with family members in the family burial cave, and an extended family would have one burial cave with limited space thereby necessitating this practice. we find, for instance, that the Me'aras haMachpeilah only had room for four couples and it was considered an honor of the highest order to be buried there.
The Shulchan Aruch stipulates that there are only three reasons to allow disinterring a Niftar: 1) to be buried next to family members, 2) to be buried in Eretz Yisrael, and 3) if there is a liklihood of the Kever being damaged (see YD, 363:1). Even when Likut Atzamos is allowed, the skeleton and remaining sinews must be kept intact (YD 403:6) (this is in stark contrast to the practice in other religions of separating the skull and other large bones and placing them in their respective piles in large communal ossuaries).
This is not a Psak Halachah