Why put 158K resistor between the input signals and negative input signal and ground?
The designer was thinking about the potential problems that might occur if the sensor was disconnected.  The Instrumentation Amplifier would have floating inputs - this can lead to all kinds of issues.

Remember that Hall Effect sensor (or bridge) is relatively-low impedance.  The designer chose resistor values that are high relative to the output impedance of the sensor.

Also note that there are two ways to connect those added resistors: as shown in the diagram or from each input to Ground.  The designer chose to do it the way (s)he did, as a guess, to reduce possible error caused by the resistors being in the circuit.  Again: look at the offset that is introduced by the ratio of the added resistors compared to the output impedance of the sensor.

In my opinion, these added resistors are a sign of good engineering practices.