Life eating other life.

[Anomalocaris]

Intelligence depend on complex behavior.   Complex behavior and predation go hand in hand.    But any Complex behavior on the part of autotrophs would likely be driven by predation avoidance.   Predation itself require behavior more complex than simple autotrophism.

But more subtlety, motility and complex behavior in autotrophs, even if not driven by predation, would tend to open up better better opportunities than autotrophism for the former autotroph, thus again reinforcing the association between predation and complex behavior.