[Avodah] Likut Atzomos

[Akiva Blum]

The Mishna in pesachim perek 8 mishna 8 mention someone who is melaket atzmos oviv, gathering bones, as tomeh. This also appears in perek 1 of moed katan. The Tiferes Yisroel in pesachim says that people would bury their dead in a temporary grave, and after the meat had rotted, the bones would be reinterned in their ancestral cemetery. Can anyone point to a source, and perhaps also an explanation for this practice?