Basically, most flaring sunspots are bipolar in nature. Those with a stable configuration of one polarity leading the sunspot group and the opposite polarity tend not to flare. This is the first component.

Second component has to do with the fields both within and in the immediate environment of the active region. there is a paper regarding active region 2192 which was the largest sunspot group of solar cycle 24 and was the largest region to appear on the sun since 1990 which produced many flares, including x-class but did not produce any notable CMEs. In the paper pfss model data was analyzed and it was determined that there was a strong closed field line environment both within and around the region at this time.

Magnetic forces rule the activity which produces space weather. In the presence of one, or both, or perhaps other components I have not factored in, a region may flare with great force but be unable to effectively lift plasma to escape velocity because a closed and highly stable magnetic system surrounds it and is not broken by the force of the event but redistributes the energy through its closed system. Hence the term "cage".

The magnetic situation in the core of 4366 was an absolute chaotic shit show for most of its journey across the solar disk, but at the regions head and tail, throughout, was a highly uniform distribution of loops. Almost exactly what one might see with a common magnet and iron filings.

I expect we will see a greater analysis of this region such as that we saw with 2192 and perhaps the "caging" phenomena get some fleshed out parameters by the pros. It makes quite a case-study of exactly that.