Sounds like he was likely a high SPD, low STR LB who wasn't even suited to the 3-4. Scouting likely would have shown you that he probably had a STR rating below 70 (not ideal for a LB), and a SPD rating above 86.
If you want, move him back to LB, he should stay B potential now though. That's a good way to break Potential limits. If you can swap a player to another position where his OVR will go up, his potential can also go up. Moving him back to his original position will lower his OVR again, but Potential doesn't reduce.
I've turned countless B potential players in A potential players like this. 3-4 DE to DT will usually boost. FS to SS or vice versa can cause a boost. RB to WR, or TE to WR can cause it as well. LB to DE is an easy one too (though I swear, they might have patched it, because the last few OLB's I've moved to DE to try to exploit this barely increase in OVR now).