I feel like I'm still missing one important piece of the puzzle.

The chart near the end, around latest hardfork, shows that increasing interest on HBD has strong correlation with raising hard cap price - something withesses should be taking into account.

It also shows that explicit HIVE-to-HBD conversions are extremely effective in pushing supply of HBD to the limit - a good argument for making hard limit noticably higher than upper soft limit.

However there are two other places where hard cap price rises: near 13M mark (HF19) for a short time followed by period of steady price and later between 19M and 23M (all before HF20). Such rise of hard cap price can only be caused by either falling supply of HIVE (did not happen as we can see on bottom chart) or raising supply of HBD (not visible as spike in virtual supply because in the same period market price was very high). Since in that time interest on HBD was zero, proposals were still not a thing and there was no HIVE-to-HBD "on demand" conversion what process caused that effect?