The easiest answer would be that yandere-chan enlisted truck-kun's help to remove the competition. Would be a bit too predictable if that's the case though.
1. The girl was possessed by yandere girl and got hit by the new enemy
2. The girl was not possessed and yandere girl hit her with the truck
3. The girl was not possessed and the new enemy hit her with the truck
Yandere girl could actually work with the enemy to get rid of competition too
You all saw the words 'rip' with thr frivers hand on his neck. Thats a sticker undoughtingly. Yandere bitch killed her, we all knew it was gunna happen