Blow primers are a sign of dangerous pressures, correct? Check the primers on rounds that you've previously fired or shoot at least a few more S&B's. Check to see if the primer is flattened out (matches the same height as case bottom). If it's not nice and round looking around the edges, you MAY have some hot loads.
Another cause would be bad head spacing. If the head spacing is off, the bullet can be seated into the lands of the rifling, which can possibly increase chamber pressures to unsafe levels.