Placing moulds directly on shelves can cause breakage as my kiln keeps reminding me. If you put a mould directly onto the shelf, it apparently keeps the kiln from evenly heating the shelf on the way up and the mould keeps the shelf hotter than the edges on the way down. I don't know whether the shelf breaks on the way up -although I think that is so - or the way down. It doesn't happen every time, and that's why I forget.
It seems there is a critical relationship between the size of the shelf and the size of the piece covering the mould. The greater the proportion -up to some maximum, maybe 90% - the greater the likelihood of breakage it seems. A fully covered shelf would heat and cool along with the mould. When the mould is small in relation to the shelf, the heat can travel under the mould well enough to avoid breaking, it appears. It is the large range in between that causes the trouble.
A preventative is to fire without a shelf. But failing that possibility, raise the mould a little from the shelf with kiln furniture or pieces of thick fibre paper. Also keep the shelf elevated a little from the floor of the kiln.