illustration (attribution, if any possible, is at the end of the article)
When asking "Were Mahasanghikas promoting a major departure from the early teachings?" (gplus.wallez.name/DmpzDdK3Kr6), one may ask what is the Abhidharma or what are the Abhidharmic schools. I don't think this is at the heart of the Mahasanghikas' split, but this remains an interesting question. What is the Abhidharma?
The tradition asserts that the Buddha, immediately after his enlightenment stayed meditating and polishing his teachings under the bodhi tree for a few weeks. The insights and thoughts of this period would be the Abhidhamma, later taught to gods and then to Sariputta.
This traditional account would consider that the Abhidhamma is either directly the word of the Buddha (orally transmitted like the sutapitaka and the vinayapitaka), or at least teachings causally arising from the Buddha (i.e. the Buddha taught a few disciples, knowing full well the sort of conclusions these disciples would come up with at a later stage).
Modern theories would rather suggest a systematisation that occurred about a century after the Buddha's passing, mostly based on a method of analysis that the Buddha himself had used (but led independently by the strongest teachers of the time).
The systematisation includes the results of analysing further some phenomena, e.g. in an attempt to find what's key in discernment or what are the critical steps on the soteriological path; but it also includes the results of grouping, categorising, organising, summarising the teachings (e.g. to provide a theme-by-theme approach to the sūtras). The latter fed the former, because organising the teachings in a systematic manner helped un-cover which phenomena had been a bit less thoroughly examined (or at least spoke about) by the Buddha.
The matrices and summaries are easily neglected by modern scholars but they played a key role in helping teachers spread the Dharma. However, by their very nature of summary, some actually opened the possibility for drifts in interpretation.
A taste for symmetry could also have played in role, in pushing teachers to 'complete' matrices (even when it was of little practical value).
Drifts in interpretation have, in and of themselves, played a major role in the further development of the Abhidharmas, as various schools not only recorded their interpretations but also elaborated responses to the theses of other schools (sometimes with a pre-defined, clear agenda of distinguishing oneself from others rather than of 'objectively' finding the truth).
For example, the analysis of "full cognition" as 17 moments of thoughts / 1 moment of matter in Theravāda is a direct result of looking for alternatives to the Sarvāstivāda and Sautrantika doctrines.
Moreover, sections of the Abhidharmas could be dedicated to record official doctrinal positions in controversies with other schools (e.g. the Kathavadu of the Theravāda or the Vijñanakaya of the Sarvāstivāda).
The resulting proliferations of views gave rise to the Sautrantika school, as a reaction to the multiple interpretations based on "later analysis" rather than on the sūtras.
But the Sautrantikas were not opposed to the further analysis, they mostly wanted to ensure that the sūtras served as anchor and thus they did not accept the same "authoritative" status to the Abhidharmas.
The Madhyamaka school also arose as a reaction to the Abhidharmas, potentially trying to bring back monks lost in mental interpretations towards a more soteriological understanding of the Path, thus mounting an 'attack' on all dharmas now uniformly declared empty of essence, empty of own-nature, etc.
Today, the Abhidharmas (better used in combination rather than just from one school) remain fantastic tools, for some, to nurture their growth and support their enquiries, marvellous ways to raise questions and support curiosity… They're also a fantastic opportunity, for others, to cling to certainties and predefined 'truths' which they'll rely on with argumentum ab auctoritate fallacies (in plain contradiction with the kalama sutta gplus.wallez.name/PUQ2AeReGEM).
If you fall for the projection that the "truth in ultimate terms" is an "ultimate truth", higher than the "truth in conventional terms", then you're likely to fall for the projection that some Abhidharma or another is a "higher teaching" than the sūtras themselves. Avoiding extremes is more constructive.
image: (traditional account) The Buddha preaching the Abhidhamma to his former mother, now a Deva, and others in Tavatimsa Heaven.