You have an ideal current source (I1) in the source connection of M6 and M7. A current source in Spice will draw the designated current no matter what the rest of the circuit does. Here it is creating a negative voltage to generate the required current because the gate voltages at M6 and M7 are not high enough to generate the 0.5mA current value (what it the threshold voltage of those?). Try increasing those gate voltages.
Since an ideal current source is not a real component you should substitute a real current source using a transistor. Then your bias values will be more realistic.