Finding it hard to follow your train of thought, but if I understand you right...
There are multiple factors, not one "root cause". Orbital precession isn't enough by itself, but when combined with orbital eccentricity and obliquity AND favourable topology, then you get an ice age. That's why they don't occur at *every* orbital cycle - and why they can sometimes occur between cycles (e.g. if intense volcanism causes enough cooling to trigger an ice age by itself).
If you want a specific example, try this paper, which describes how, 116,000 years ago, a pattern of ice sheet formation and melting every few thousand years was triggered by the Bering Strait being shallow enough that whenever sea levels lowered sufficiently through ice formation, the Strait closed, which changed the salinity mixing of the Pacific and Atlantic oceans. This intensified the Atlantic's meridional current, which warmed parts of Greenland and North America sufficiently to melt enough ice to re-open the Strait - and the pattern repeated.
This pattern was eventually broken 34,000 years ago when (yes) we reached a point in our orbital cycle that kept temperatures cool enough, and the Strait closed long enough, to stabilise the climate, so that when it opened once more 10,000 years ago, the climate remained stable enough to allow our civilisation. So as you see, it's not so simple that there's a single "root cause" we can pin it on, but that doesn't mean we don't know what did it - we can see (and simulate) how multiple factors combined and interacted to result the ice ages we can see in the ice core record, which gives us a pretty solid explanation as to the causes of all the ice ages over the last 116,000 years.