Why is there spikes in the negative moment graph? 

The spikes in the negative moment is due to definition of the "lane support negatve moment", as the software applies the AASHTO clause 3.6.1.3.1 for negative moments is calculated only at the nodes. The bending moment calculation for the element is calculated for 5 points that is at i , 1/4, 2/4, 3/4 and j end but the magnified moment that would appear due to lane suuport negative moment is calcualted only at the i and j ends. Thus at those location we see the spike in moments  and not in the intermediate location.