Their Questions section includes:
What caused the snowball earths?Possibly by a lowering of atmospheric greenhouse gases to near-present levels through tectonically-mediated rock weathering, when the Sun was considerably dimmer than present.
Well, that's the short answer - here's the longer answer.
The Wikipedia article on Snowball Earth is also a really nice summary of the topic and covers the evidence for the theory, the various different controversies and, most relevant to us, a discussion of how life itself might have survived, and the implications for evolution.
We know more about what causes regular ice-ages but even then it seems that the relative contribution of different factors is much debated.