The most likely cause is rolling shutter interacting with helicopter vibration. Phone sensors are usually read line-by-line rather than all at once, so if the camera vibrates while the image is being scanned, straight details can be recorded in slightly different positions across the frame, producing a wavy or rippled look.

Helicopter rotor wash can also contribute. The spinning blades create strong pulses and turbulence in the air, and changing air density can bend light slightly, similar to a heat-haze or mirage effect. That can add distortion, especially when shooting distant subjects.

So the effect is likely a combination of very strong vibration plus the phone’s rolling shutter, with turbulent air sometimes making it worse. It appears only on some frames because vibration, rotor pulses, shutter timing, and scene details don’t line up the same way on every shot.