Villagers will attempt to walk under trapdoors despite their hitbox being too large, causing them to get stuck walking against a trapdoor despite there being other paths they could take.

To replicate this you could build something like I have shown below. When they want to go inside they'll often get stuck on the trapdoors under the roof. They will not stop trying to walk under the trapdoors until I open them.

EDIT: this also happens with trapdoors on the ground and two trapdoors layered ontop of eachother, with one being on the cieling and one being on the ground.