Let us take the example of the MOSFET with open drain.

There are 2 cases:  
1. When VGS << VTH, the drain terminal is indeed a high impedance floating node
2. When VGS >> VTH,, the drain terminal is not a floating node. In this case, there is a low impedance between the drain and source. 

[![enter image description here][1]][1]

Assume initially VGS << VTH and the drain is sitting at 0V according to your simulation without pullup. Basically, you can visualize this situation as a capacitance across drain-bulk junction as shown which is holding a charge of 5V (drain voltage = 0V and bulk/source voltage = -5V).
Subsequently, when VGS >> VTH the drain-source becomes a low impedance and hence discharges the cap fully. Thus, the drain voltage becomes equal to the source voltage i.e., -5V. 

**Essentially, the pullup resistor is only needed to pull the drain high. To pull the drain low, we just need VGS >> VTH to be satisfied. If there is no pullup in this case, the drain capacitance will get discharged and eventually drain voltage = source voltage**

  [1]: https://i.sstatic.net/K1kDQ.png