Why the MOSFET VDSS is measured at 250 µA?
The number isn't significant on its own...VDSS indicates the drain-source voltage that won't damage the part.  As the voltage is increased (based on an enhancement mode device with zero gate-source voltage) it will eventually break down and conduct enough current to damage itself.  250uA was chosen by this manufacturer as a level indicating the beginning of breakdown but not damaging to the device.  Other manufacturers specify it at other currents; for example, this device is specified at 5mA.