He's a known backstabber
One of those who plot at pleased..not an aggressive AI overall, but every turn there's a small rng chance if borders overlap.
You cannot see if AIs plot in semi-isolation, cos you cannot ask them to declare war on somebody else.
Aka no "we have enough on our hands right now", which triggers the red fist warning in BUG.
So he prolly was plotting, rarely "daggers" are also possible (instant dow) but it's unlikely here.