Is this one NPC interdicting you every jump?
Have you tried (not a confirmed fix) seeing if it's related to stacking missions? We've seen some people reporting this.
Zac, it varies. Usually it means one NPC pirate has eye on you and chase you trough system. I think it is by design, however NPC sometimes ignores damages acquired or how fast it can interdict again. It feels a bit like Terminator
I like this bit of persistence, but I think NPC has to be less successful in chasing you down every time (or at least it should scale according to it's combat ranking), and keeping damage and weapon stats consistent between interdictions would be cool as well.
It happened to me once when I had three different missions in different systems (so I don't think would call them stacked). NPC also didn't express interest in my mission but more in my cargo (I had 4 telemetry suites in cargo).