It seems to be a manifestation of tonic immobility [0] triggered as a defense mechanism in response to a perceived threat. As far as why drawing a line is perceived as a threat is anybody's guess. Maybe it's the chicken's simple brain using a flawed heuristic for recognizing snakes? It brings to mind cats freaking out about cucumbers. [1]

[0] https://en.wikipedia.org/wiki/Apparent_death#Tonic_immobilit...

[1] https://youtu.be/RBrZsgy4-SQ?si=OXD65pDCd9hqaNuY