Mostly PED suspensions, but in some cases free agents are suspended for other things.
Josh Brown was suspended 6 games in 2017 for domestic violence (after an investigation that lasted a year, even though he admitted abusing his wife before his initial one game suspension that caused a lot of controversy) and
JJ Wharton was suspended for 6 games in 2016 after an assault conviction. Is the NFL seemingly arbitrary in it's suspensions? Yes. Why did they spend a year investigating Brown for domestic violence AFTER he admitted to it in writing the Washington State Police? Were they waiting for him to get out of the news cycle, be "too old" and too far removed from competitive games to be attractive signing for a team and then give him a coup de grace suspension? But, regardless the NFL suspends free agents all the time, not just for drugs, performance enhancing or mind altering. They tend to drag their feet "investigating" high profile cases and come down hard in the off season when fewer people are looking. Brown is screwed and will likely never play another down in the NFL. I wouldn't be able to say one way or the other if he deserves it because this is the NFL we're talking about. He likely deserves it, but the NFL is so PR focused they would throw anyone under the bus to "protect the shield".