Not entirely sure.
Think it started just because he was playing poorly for quite a large chunk of last season.
Then he reacted to it at some point when he was whistled when getting subbed.
Don't know if anything has happend over the summer aside from Insigne's girlfriend making some stupid comments.
My guess is that it is just the issue of Insigne being the sole Napoletano in their squad. That leads to tensions because the fans expect more of himm and the player takes things more personally.
Wonder if that is why, despite their being heaps of footballers from Naples, very few end up playing for Napoli?
The Aberdeen support is really bad for holding local players to higher (almost impossibly high) standards compared to players who have no emotional attachment to the club.