Talking of motile plants, maybe we can use the walking forests of South America as inspiration. There is a type of tree I know as the Walking Palm (idk if that is its actual name, or if it is even a palm tree) that, over the course of a few years, sends out new roots to a spot with more sunlight, lifting up its old roots and moving the whole tree inch by inch to the new spot.
A species such as this could be the ancestors for moving plants, maybe a species in a jungle or similar area with still some access to sunlight but mostly blocked by the canopy. This small plant uses it’s roots to move to areas of better sunlight, but it also lacks some necessary resources to grow because of the abundance of plants in every area drawing in the same resources.
As such, it must find a new food source, and one option would be to become a scavenger. Send roots into nearby carcasses to absorb nutrients before they are absorbed into the ground and into other plants. To do this more effectively, it would need to become faster, getting to the carcasses before other scavengers get to it or it decomposes. To do this effectively, it would likely need another source of energy, and might begin absorbing more and more of the scavenged creatures for energy as well as the nutrients it needed in the first place.
This could potentially lead to more and more motile plants to try and find carcasses before they can decompose or being absorbed by other organisms. They would still use photosynthesis in between hunts for carcasses. This new source of energy could also allow for a new abundance of energy, like that required to have higher thought for example