Table 6-1 "Data Phase Control Signals for Targets" in the PCI32 User's Guide describes 4 conditions for a target application based upon the values of S_TERM and S_READY. The second condition, "Normal", states that STOP_IO is value logic high in this condition.
It is possible that STOP_IO may actually be logic Low, despite S_TERM being Low and S_READY being High.
In a circumstance where an invalid address location (i.e., one that does not end in a 0, 4, 8, or C) is supplied to the target, the target may assert STOP_IO to indicate an invalid attempt at a transaction.
This is one of various possible conditions that might cause STOP_IO to be asserted.