I hate to break it to you, but Post Malone doesn't have long hair anymore. Although we don't have much insight into why the haircut was "manufactured," we can spend all day exploring our own psyche, and in the end, we'll come up short. Maybe Post Malone has a wedding to attend, a passport photo to clean up for, who-knows-who cares? Well, I'll tell you who cares: the people that liked him just the way he was.
Kidding aside, it seems Post Malone was reacting to two big dreads forming on his scalp. The knots were likely at a "point of no return," so he was left with little options but to bring the house down. In his short existence as a viable pop star, we've never known Post Malone to rock short hair. In fact, we've even grown to accept the "stinky" air about his appearance, mostly due to his shaggy hair. The costume department has done an excellent job of working around those innate features.
There's also a theory that his haircut is predicated by a paradigm shift occurring in his career, from B+ to A or A+ status, in eyes of the public. Although is pretty uncharacteristic of his personality, or what we know of his personality, we should take a moment to congratulate him on crossing yet another threshold of relative success, notwithstanding the haircut which has been central to this conversation.