Frogs often get stuck beneath lilypads.
To reproduce
Spawn some frogs near a one block high water pond with lily pads. Eventually they will get stuck there.
Alternatively just place a lilypad and spawn a frog with a spawn egg underneath.
Video
2022-03-16 23-20-36_Trim_Trim_Trim_Trim_Trim_final_FINAL_v3.mp4