To answer this question we go to the Ediacaran animals in deposit on the Avalonn Peninsula in Newfoundland. The community is from a deep seafloor environment, several hundreds of meters deep, from the northern edge of Gondwana. Analyzed samples suggest deep-water anoxic conditions persisted before 580Ma, shifting toward a stable oxic period of a least 15 million years. The process is bit complex so as a list…
1) This shift in oxygen concentrations corresponds with the Gaskiers deglaciation
2) Glacial melting increased nutrient loads in the ocean.
3) This in turn promotes primary production and carbon burial
4) This increases atmospheric oxygen levels.
5) Which allows for oxygen concentrations large enough to allow for respiration of large, multicellular organisms.
Summary of and figures from Canfield et al. (2007, Science)