How was the Marinoan Glaciation triggered?
Several articles suggest that the mechanisms involved with the weathering and/or the breakup of supercontinents as being a mechanism for this kind of glaciation, in the timeframe pertinent to your question, Rodinia.

According to the article Precambrian supercontinents, glaciations, atmospheric oxygenation, metazoan evolution and an impact that may have changed the second half of Earth history (Young, 2013), a suggested mechanism of the onset of glaciation is the formation of the supercontinent, specifically, the author postulates:


  Enhanced weathering on the orogenically and thermally buoyed supercontinents would have stripped $CO_2$ from the atmosphere, initiating a cooling trend that resulted in continental glaciation.


The Polarisbreen Formation is touted in the article The Marinoan glaciation (Neoproterozoic) in
northeast Svalbard (Halverson et al. 2004) suggests that weathering as being one of the main factors for the sustained glaciation.

An alternative hypothesis is proposed in the article From Volcanic Winter to Snowball Earth: An Alternative Explanation for Neoproterozoic Biosphere Stress (Stern et al. 2008), where the authors suggest that increased global volcanism from the breakup of the supercontinent.