Spike on graph generated
Before posting such question its actually a good idea to isolate the problem. For that you should throw away all but the resistor ladder, and you can see the same problem.

If you now zoom in at the point of one of the spikes, and show all voltages of your voltage sources, you will notice that the source that ramps down only starts doing so once the others are ramped up.

As Trevor says, this is more or less an inherent property/problem of resistor ladder based DAC designs and will happen to a degree on real implementations too, as the timing of the switches will never be perfect.