Breaking down that number a bit:
He's -3 at even strength, where he's played softer minutes than earlier years and performed worse. This has been regardless of who has been on his line, even though Brown has been almost a constant. Main issue is not defensively, we just don't get scoring from that line as can be seen both in how much that line creates and how well they execute on those chances. That's hardly a surprise when you have a Kadri who sees goalposts in his dreams while largely playing with two offensively limited players like Lindholm and Brown. We've seen what such wingers can do even to offensive talents like Matthews and Nylander.
Another minus is from being on the ice for a shorthanded goal. The remaining four comes with the net empty.
To summarize: Over half of those minuses says little about his play. The rest are a combination of Kadri slumping personally, and mostly being forced to do pretty much the whole job in transition and chance generation, leading to a ineffective line.