Why is there a negative value on the scada when the analog input wire is disconnected from a field instrument?
It's common in control system design to want to indicate a faulty sensor in a positive (unmistakable) manner. In an analog system the only way to do that is to send a signal that's well out of the normal range of measurement. An input of zero (faulty sensor) could otherwise be confused with a valid input. 

In the case of temperature transmitters used in heating applications, for example, we usually send a signal that is over-range high, so that the heat will shut off. In some applications you want the opposite. 

Also it might be worth noting that in a system with 4-20mA transmitters, an open wire will represent an input of -25% of full scale for a zero-based signal. So if 4mA = 0°C and 20mA is 100°C then open (0mA) is -25°C.