Many (but not all) prayer customs are just that - customs. Customs develop organically, without a conscious "Authorized Halakhic Rationale" serving as the reason why they should be enacted. They just develop - for sociological, or impromptu, or it-just-feels-right-and-makes-spiritual-sense reasibs - and if they organically catch on, well, then its a custom!
We can certainly guess as to the sociological reasons why standing for these mishebareich's developed and spread (way of expressing these prayers' particular spiritual importance in the lives of the congregation). But we would BE NAIVE to assume that one day someone got up and said: "For this particular reason and that particular reason we should now start doing X."
The same can be said for standing/sitting for kaddish, or answering Amen vs/ Brich Hu, or standing up during the end of Tachanun. Unless you have good reason to think that early on someone got up and gave a guiding reason, the minhag probably just developed organically.
After the fact, we can and should "read" these customs for the values they express. After all, these customs express the holy "Intuition of the Masses" of the Jewish people. No one got up and gave an explicit reason to start things off, but the community as a whole were attracted to some grammatical/spiritual/liturgical/halakhic truth which made them - as an organic mass - develop a given practice.
So too with standing/sitting for MiSheBareich's. Without any direct Halakhic order from above, the Jewish people are organically sensing certain liturgical/spiritual truths which make standing up obvious and appropriate for at some times, and less necessary at other times. If the custom really spreads and solidifies, then in a few centuries, expect Rabbis to look back and try to "read" these customs for the values they express.