I suspect this is one of those questions where the real answer is lost to history.
If my memory serves, Aurelius devalued his coins possibly owing to substantial military activity against the “barbarians”. There was also a rather nasty plague outbreak early in his reign too.
So poorer quality coins for a reduced population?
People would want to get rid of them compared to good quality earlier pieces. So perhaps they were more frequently exchanged & thus worn out.
Plus he was just before the year of the five emperors. Maybe they reclaimed as many coins from circulation so they could melt them down & slap their face on it.
As speculated, these lower desired coins would be the more frequently traded & thus end up in “banks” or back in the “tax man's coffers” so could be more easily withdrawn. Pure speculation here though.
I don't know about mintage. Usually emperors wanted to slap their face on every coin, statue etc as soon as they took over so everyone knew who the new boss was. I can't imagine he was any different in that respect.
