Why do birds that walk on the ground walk so "bumpy"?
If you mean the head bobbing displayed by many birds when walking: it is believed to help with visual stabilization.


  The term head-bobbing, used for the apparent back and forth movement
  of the head of some birds while walking on the ground (think pigeons),
  is a bit of a misnomer. These birds don't actually bob their heads
  back and forth – instead, they move their body forward, leaving the
  head behind, then thrust their head forward past their body (hold,
  high-speed thrust!). This innate behaviour is believed to help several
  key aspects of vision during locomotion and foraging. While the head
  is still, images are stabilized on the retina and moving objects
  become clear against the background, and during the thrust phase,
  depth perception becomes more accurate.


https://jeb.biologists.org/content/jexbio/217/11/1836.1.full.pdf

In this article you can find an stroboscopic photography that shows the particular bumpy walk.