What could cause a turn from global warming to global cooling?
Pleistocene's climate variations are well correlated with Milhankovitch Cycles. 


  "Variations in eccentricity, axial tilt, and precession of the Earth's
  orbit resulted in cyclical variation in the solar radiation reaching
  the Earth, and that this orbital forcing strongly influenced climatic
  patterns on Earth."
  
  Source: Milhankovitch Cycles, Wikipedia


At this graph you can see eccentricity explains major Oxygen $δ18$ variations at Pleistocene oceans. $δ18$ is a known proxy for temperature.


   
  
  Source: euanmearns.com


The minor peaks are Earth's internal forces reaction to changes on solar radiation. Earth has subsystems that store and interchange the energy received from the Sun, resulting oscillatory climate tendencies.

Aswell as the atmospere and the hydrosphere, cryosphere has internal forces as a subsystem; and oscillatory events (development of ice-break of ice). One example of punctual changes due to cryosphere internal forces dynamics are Heinrich Events, that can influence climate at minor scale than eccentricity does and be correlated with some of the peaks your graph shows.