This is the bizarre thing - the only time I got interdicted was soon after picking up UAs from a crash site, as reported by others, but that seems to be the only thing that FD have specifically said is not the cause (on Galnet):
In his statement, Admiral Tanner said:
"Having reviewed all available information, we can confirm these craft are active primarily within the Pleiades Nebula and the space in and around the Maia and Merope systems. The significance of these locations – if any – has not yet been determined. We can also confirm, despite rumours to the contrary, that carrying an Unknown Artefact does not attract the attention of these vessels."
I know it can't be barnacle related as I never visited any until after the interdiction (unless 2.3 changed the rules of course).
Struggling to think of anything else that could be the trigger, pure RNG would be ... much disappoint
Hmm, well, here's a thought - let's look at that Galnet quote again:
carrying an Unknown Artefact does not attract the attention of these vessels
Carrying is only one part of the interaction we have with the UA. First, though, we are scanned - and it draws our ship. Perhaps... we are being tagged?