For one thing, you're not observing a trend so much as pointing out a few late round picks doing well last year. How does Zac Stacy and Andre Ellington have anything to do with Devonte Freeman? Two guys does not constitute as a trend.
I agree with this point. But I also think there are a couple trends that we have seen.
First is the general devaluation of the position. It's not surprising to see later draft picks or lower paid journeymen do well when the league just isn't paying top dollar for running back talent anymore. Maybe Jeremy Hill and Eddie Lacy would have been borderline first rounders five or ten years ago. Now they slide to the second. Maybe Freeman, at 5'8" and 208 lbs with 4.56 speed, isn't that far off Frank Gore at 5'9" 217 running a 4.58. Gore was an early third rounder. Freeman was an early fourth. With the success of the second round running backs last year, it's reasonable to say that good talent is going cheaper than it used to. It would be a fallacy to say that that means Freeman is good. But it's every bit as much a fallacy to say that his draft position means that he's not.
The second is the more specific case of smaller running backs succeeding. Zac Stacy is 5'8" 216 lbs and ran a 4.53 40. Ellington is 5'9" and bulked up to 210. He ran a 4.54 40. Ray Rice is 5'8" 212. He ran a significantly faster 4.42 40. LeSean McCoy is 5'11" 208 lbs and ran a 4.5 40. There's some fluctuation here, but all of these guys are relatively small and have held up to the pounding. That doesn't mean that every little guy will succeed in the NFL, but there are a lot of guys under 5'10" and under 220 lbs running the 40 in more than 4.4 who are succeeding. We might prefer if he were a 4.24 40 like Chris Johnson, but people don't seem to think Johnson is all that great these days, so maybe we wouldn't.
The third is that teams are using a few distinct strategies to staff the running back position. A lot of the better backs in the NFL are stuck in committees right now. There are three quality backs in Buffalo, a couple very good ones in Cincinatti, as many as three very good ones in Seattle, a bundle in San Francisco, a few more good ones in New England, and so on. How does that happen? Some teams are stockpiling talent at the position, while others are grabbing one stud and hoping he stays healthy, and others are willing to go cheaper at the position and hope that their guy can is good enough to shoulder the load they're given. I'd like to say that Freeman is one of those true studs, but there's no reason to believe that yet. There's also no reason to believe he's on a loaded team. SJax is getting old, and is already hurt. Jacquizz was briefly tagged as the heir apparent, but he hasn't been good enough to hang onto the role. Unless he pans out to be a diamond in the rough, the most likely case is that Freeman's on a team that's gone cheaper at the position, and is hoping that he is good enough to shoulder the load. They seem to believe he has lead back/feature back potential and have said so repeatedly. I tend to agree - he can block, catch, run inside, run outside and score TDs. I don't need him to be elite at all of those things, I just want him to be good enough to keep getting fed in all the ways that give me fantasy points.
I don't have a crystal ball and I don't know how good Freeman can be, but I know that running backs have short careers, and I'd rather have a guy who gets his opportunity early than a guy who might not get it at all (Lattimore, Turbin) or might not get it until much later in their careers (Gerhart, McKinnon) or might be considered one dimensional (Helu, Blount, Andre Williams). What I like about Freeman is that he appears set to get that opportunity, and that he's capable in all of the dimensions that get him on the field and keep him there to get receptions, yards and touchdowns. He still has to do well enough with that opportunity to keep the job, but I'll gladly take someone who has the opportunity over someone who doesn't, and there's a very short list of young running backs who have that opportunity. If he pans out, he's a valuable, scarce commodity.