This is very interesting to me, so I'll break down my reasoning.
Steelers cut Boykin after showing some promise in our system.
The Panthers pick him up and could really use a quality slot corner.
Boykin was rated as the top slot corner 2 years ago.
Before training camp even starts, Boykin is cut.
You do not cut a quality corner that can help your team in a position of need at a bargain basement price, unless:
1. He's about ready to be arrested for a crime that would warrant bad press or a suspension.
2. He's about ready to be suspended for violating the NFL banned substance policy. (As SF81 touched on).
3. He has a serious injury. (Though the Panthers would have had to give him some injury compensation to drop him, so I find this option very low on the probability scale)
When Boykin was cut. The Panthers had to eat the $80k signing bonus (not a ton of money, but still it is now dead money). If he was suspended, it would have been his first offense, and would carry a 4 game suspension. I don't personally think you cut a player for violating the league's substance abuse policy, when he'd have the ability to still practice and learn the defense all preseason, only to sit out one month. You'd get a player back, and healthy, and ready to play in October, and this is something that a SB caliber team can roll the dice on with little risk.
My thought is something big is about to come out about him. You don't cut a new player that can help your team when he'd only be missing 4 games. You cut a guy to distance your team from some terrible news that's about to break about that player.