It is consistency, a lot of it is mental, and some is luck and sequencing of events. He struggled both physically and mentally from the stretch (part of why I don’t think he is a reliever), and just got hammered with runners on base (a lot of parallels with Jesus Luzardo). This goes back to college, but Griff can get emotional and overthrow when things go poorly, and the catchers and he got better at pausing and refocusing (if you look at logs you will see times he walks a guy and then immediately loses the zone before then striking out the next few guys). As we saw with Luzardo, the best way to deal with struggles with men on base is to prevent them from getting on.
This next bit is a bit more anecdotal, but he looked very good to start, then had the minor injury and it took a bit to get everything in groove. He re-added his sweeper this year (to the cutter and curveball) and by the end of the year it was a monster pitch. So I think down the stretch you saw a guy who started to have things click more, built some confidence, and that started to mitigate his largest weakness.