Cracks on the bottom of slumped bowls initially appear mysterious if not impossible.
It seems that what is happening is that the crack results from too fast a heat up.
The top gets plastic during a - relatively speaking - too quick a rise in temperature, while the bottom is still too cold to move. The piece splits on the bottom to relieve the stress of the weight of the upper portion of the glass. It does not break completely through, because the top is hot enough to move rather than break.
This bowl split as it was much thicker than I thought. The firing was too fast and the top began to slump before the bottom was warm enough to move. This is an extreme version of the split described above.
You need to put much more heat into the piece in the 50C above the annealing temperature if you experience this splitting effect. This can be done by a soak in the region of 540C-600C for Bullseye, or by a very slow rise in this region. The heating above this also needs to be slow to ensure the glass at the bottom of the piece is nearly the same as the top.
You will find that as a result of these slow rises or long low temperature soaks that the slump will occur at a lower temperature than with a rapid rise. This will also have the additional benefit of leaving fewer mould marks on the bottom of the piece.