Hard to say what’s going on without some more Khushroo,
but it sounds like either too much old metal, the metal is being
over-heated, the flask temp may be too high or it might be that you
are quenching it too soon after casting. Or maybe it is being caused
by a combination of some or all of the above.
If you can post a little more info, like what metals you are using,
whether this is a new thing or has been a problem for a while,
whether the cracks go all the way through the metal or are just at
the surface, or anything you may have changed recently, like a new
batch of investment or a new kiln or torch, we might be able to be a
little more helpful.
At any rate, I hope this helps!